Size of the vicarate
In the books it has been established that there are 300 Vicars, but that seems a bit many to me. I mean there is but one archbishop in Charis, and when you consider the percentage of the population, and more importantly the wealth in that less prestigious archbishopric there is no way there are even 100 archbishoprics, probably less than 50 for that matter, considering that the temple lands are probably divvied up among the vicarate. While I'm sure there are more of them in the temple lands itself servicing the bureaucracy having enough of them to create a reasonable ratio would mean they hang around like flies, and that 90% of the vicarate has nothing to do.
You do remember that 85% of the population of Safehold lives on the "mainland" continents, don't you? That means that only 15% of the entire population lives in Charis, Corisande, and Chisholm combined. The mainland countries have much higher populations, which means that they have much higher numbers of clergy, which means (surprise!) that they also have much larger numbers of — roll of drums — archbishops. In fact, most provinces of the mainland realms have their own archbishops, with the archbishop of the "capital province" serving as the senior prelate for the entire nation. Some of the most densely populated provinces actually have more than a single archbishopric. For example, the Republic of Siddarmark alone has something like 20 provinces, of which the majority actually have more than a single archbishopric. Glacierheart, Archbishop Cahnyr's archbishopric, is a mountainous, relatively thinly populated, poor province. As such, he was the entire province's single archbishop. Old Province, the province directly around Siddar City, has something like eight times Glacierheart's population with a proportionately higher number of archbishoprics. And you don't even want to get into the number of archbishoprics in the Empire of Harchong! Each of the small states between the Republic and the Temple Lands has its own archbishop, as well, and there are numerous archbishoprics in the Temple Lands themselves.
Charis, Corisande, and Chisholm were all "second-class" realms as far as the Church and the mainlanders were concerned. They were assigned single archbishops as much as 200 or even 300 years prior to the books, when their populations were still lower in both absolute and proportionate terms, and the Church doesn't worry about regular censuses and reapportionment of sees on the basis of population. Especially not when there's as much prejudice against being posted to the "out islands" as has been the case. For that matter, the various archbishops in those "out islands" have had a very strong vested interest in not having "their" archbishoprics broken up into more numerous, smaller archbishoprics, since doing so would have considerably diluted both their own wealth and the degree of power they wielded in Zion as the sole archbishops of their private domains. As a result, each "out island" archbishop represented a much greater total number of parishioners, despite the fact that all of the "out island" realms combined contained only 15% of the total population.
In addition, the number of vicars is not based upon or directly proportionate to the number of archbishops. The vicarate — which was intended from the beginning to be an effective planetary legislature, whereas you could think of the archbishops as district or territorial governors — is based upon the planetary population as a whole. The number of "seats" within the vicarate was set at 300 when "the Archangel Langhorne" first organized the Church, but the vicarate's composition has been reapportioned several times since the creation of the Church of God Awaiting in accordance with a formula Langhorne also set down. The number of vicars doesn't change; how those vicars are apportioned between the various secular realms does change, and that fact helps to account (in part) for Clyntahn's reliance upon Harchong. As the most densely populated realm, Harchong has the greatest number of vicars, who form quite a solid voting bloc within the vicarate. Those vicars, by and large, are not merely terrified into compliance with Clyntahn's policies but also actively support them because of their own reactionary orthodoxy. That same representational basis also helps to explain some of Clyntahn's antipathy towards Siddarmark; as the next most populous realm, Siddarmark has the next largest number of vicars, and while they form a less homogenous voting bloc than the Harchongian vicars (in part for reasons mentioned below), they were also substantially less supportive of Clyntahn even before the Group of Four launched the war against Charis. Losses among Siddarmarkian vicars in Clyntahn's purge of the vicarate were rather higher than among those of other realms, although not hugely so.
Vicars are chosen by the existing members of the vicarate on the nomination of the Grand Vicar. In fact, it is not uncommon for a weak Grand Vicar's nominations to actually be formulated by someone else, but under normal circumstances the process is for the Grand Vicar to solicit recommendations from the combined archbishops of the realm to be represented and then (after judicious horsetrading with his supporters and the various factions within the vicarate) to make his own selection from their recommendations. Under the current circumstances, any replacement vicars are going to be chosen by the Group of Four (which probably really means Clyntahn and Trynair), and then rubberstamped by the current Grand Vicar.
The Grand Vicar's nominations are not normally automatically seated in the vicarate. Each nominee requires a majority vote confirmation by the existing members of the vicarate, and it is not unheard of (although rare) for one of the Grand Vicar's nominees to be rejected. The possibility of that happening under the current circumstances (especially following Clyntahn's purge) is probably nonexistent, however.
Note that there are no Charisian, Chisholmian, Tarotisian, or Corisandian vicars. This represents a combination of sloth, inefficiency, corruption, and deliberate oversight on the part of several generations of vicarates and Grand Vicars. Initially, there was too little population in any of those realms to qualify them for membership in the vicarate, just as there was too little population to qualify them for multiple archbishoprics. As their populations increased towards levels which would have qualified them for their own vicars, the Church reapportioned the vicarate (in accordance with Langhorne's formula more or less) by raising the population base necessary to qualify for a vicar. The truth was that the archbishops in those realms didn't want a vicar "joggling their elbows," the current vicars at any given moment didn't want to see one of their number reapportioned out of his seat in the vicarate to make room for some out island rube, and the growing distrust of the "out islands" orthodoxy lent additional force to arguments against creating, for example, a vicar for Charis.
In many ways, although for different reasons, being what I suppose you might call un-vicared suited both Haarahld of Charis and Hektor of Corisande quite well. Haarahld, for obvious reasons, didn't want someone sitting in Zion who might have a clue about the Brethren of Saint Zherneau and his own apostasy. Hektor's political ambitions and imperialistic ventures in places like Zebediah were easier to keep "under the radar" when he only had to worry about the oversight of a single archbishop and not someone seated directly in the vicarate. Not only that, but the necessary bribery cost him far less, and the single vicar for whom he might have qualified would have gained him virtually nothing in terms of influence within the vicarate. As such, both he and Haarahld benefited from the "benign neglect" of the vicarate, and neither of them saw any reason to press for a change in the status quo.
It's important to remember that while the vicarate was established as a planetary government, it was never intended to be a representative government in any present day, real life sense. That is, all of its members were to be elected internally, as part of a closed system and without any notion of those vicars being responsible to the citizens of the realm from which they were selected. The idea was that they would be representative of their realm only in the sense of being familiar with its strengths, weaknesses, needs, desires, etc., and bring that familiarity with them to the vicarate, but their function was to govern the entire planet (in the name of God and the archangels, of course) not to contend for the interests of "their" realm. As such, there was never a tradition of serving the interests of a particular constituency, and the average citizen didn't think of the vicars chosen from his nation as being "his" vicars. This is an important distinction, and one which is distinctly alien to our own notions of representative government, and it's another reason why Charisians didn't particularly worry about the fact that they didn't have a "Charisian vicar" seated somewhere in Zion.
In the last two or three centuries, the requirement that a vicar come from the realm whose population he "represents" (in the sense described above) has slipped considerably. It isn't quite a violation of the letter of Langhorne's directives, but it's definitely playing fast and loose with the intent of those directives in many ways. In essence, even though someone may technically be a "Siddarmarkian" vicar — that is, hold one of the seats in the vicarate based on Siddarmark's population — he doesn't necessarily have to come from Siddarmark at all. Langhorne never established a "residency requirement" as a qualification for the vicarate; he simply established that the Grand Vicar should solicit advice from the archbishops and senior clergy governing the population generating that seat in the vicarate. There was no specific requirement preventing them from recommending someone from outside their realm. For example, the Siddarmarkian clergy could have recommended someone from, say, Dohlar as a candidate for "their" seat in the vicarate. As the Church has grown increasingly corrupt, it has become increasingly common for vicars to be chosen more on the basis of reliability, orthodoxy, patronage (especially), and levels of bribery than on where those vicars may have come from. This is another reason Harchong is as heavily represented in the vicarate as it is. It is also one reason the "Siddarmarkian" vicars have been less homogenous; as the vicarate has become more concerned about Siddarmark, it's also become more likely to choose a vicar from someplace other than the Republic to "represent" Siddarmark.
Finally, Langhorne wanted to guarantee that the vicarate would be financially independent of the laity it was technically "representing" in order to prevent secular pressure on a vicar's pocketbook from influencing his actions and his vote. His original plan was for each vicar to be paid a stipend or salary out of Mother Church's central treasury, but over the centuries a practice evolved in which rather than paying the vicar directly, he was granted the revenues from specific territories to support him. The idea was that this would relieve pressure on the treasury; the result was to create, among other things, the Knights of the Temple Lands.
Now, not all Knights of the Temple Lands are created equal, and legally each vicar's right to the revenues he is assigned is solely a lifetime grant. That is, it isn't hereditary, can't be passed on to his descendents, and reverts to the Church to be reassigned when he dies or leaves office. Most of the "great families" of the vicarate — the families from whom vicars are chosen again and again and again (like the Wylsynn family) — have come over the centuries to hold land in the Temple Lands in their own right. In theory, the revenues of those lands could be assigned to a vicar from outside the family which actually owns them, in which case the landholder would pay "taxes" to support the vicar to whom they have been assigned. In fact, that never happens, because there's always a vicar from one of those families who — on the rare occasions when he is not the landholder himself — is conscious of the family interest and takes advantage of the other copious opportunities for a vicar to enrich himself.
The result of all of this has been to create a theocratic government which is actually an oligarchy whose membership is self recruiting, which is not subject to recall by those it theoretically represents, and whose financial security does not rest upon the support of those it theoretically represents. And the result of that has been that there's been no external corrective to the vicarate's internal decline into corruption, self-aggrandizement, and power seeking, which, in turn, accounts in no small part for the emergence of something like the Group of Four. The vicarate had become as venal and self-serving as the Roman Senate in its final days before Clyntahn came along, and it's entirely possible that Clyntahn would have used that venality and corruption, coupled with the coercive power of the Inquisition, to make himself virtual dictator of the Church even without the "external threat" of Charis. That was certainly what he had in mind, at any rate.