Which religion fits well for Calibim? Viewed by Lore
Most vampires are not very religious, but are willing to use religion as a tool.
The best religion for the vampires would be The Occis, a cult of elite assassins devoted to Aeron. Of course, that is not present in the game.
The Stewards of Inequity would also be a good fit, as vampires are narcissists and Mammon prefers followers who believe that they worship only their own self interest. That religion is not present in the game either though (expect for being referenced in a random event).
Of the religions present in the game,
The Council of Esus is definitely the best fit for the vampires.
Flauros in particular likes to ally himself with Esus, as Lugus is their mutual enemy. Flauros does not care much for demons or the Ashen Veil (why give souls to hell when you can devour them yourself? why help destroy the world if you want to live forever?), but in the scenarios he (or at least Decius, when he is working for Flauros) does agree to work with the Ritualists against the Mercurian threat.
I tend to think that the Octopus Overlords may be the preferred religion of the Calabim chattel. It appeals to superstitious peasants, and makes it easy to abuse them. Most of the believers of this faith are pretty decent people, but the leaders are quite evil and use it to exploit them to commit atrocities. Tthe Tower of Complacency is something a vampire lord would want to keep his slaves in line ( even though it does not synergize well with the Governor's Manor as far as game mechanics go). The City of Prespur is canonically on the coast, near the Maelstrom which in Kael's D&D campaigns was a portal used by the demons of Mammon serving Hastur. (It is much less clear whether Prespur was actually under Calabim control most of the times it is referenced though.)
The Tale of Saverous mentions that the Burnt Priest (who is very much associated with the Overlords) razed the city of Prespur, and had all of is inhabitants killed. This could imply some animosity between the vampires and Overlords. The again, it is rather explicitly stated that the Overlords have great animosity for each other. Vampire lords could very well support some Cultists despite hostility of the greatest OO priest. (It is possible that Prespur had been a Bannor city at the time, or that its Calabim overlords were feigning allegiance to Junil. The Bannor holy army that finally killed the Burnt Priest did so in retribution for the destruction of Prespur.)
The Ashes of Brigdarrow contains an allusion to Danalin being with Trenton Majosi during the miraculous flooding the Calabim City of Anaea. I think it is implied that the God of Water was an enemy of the vampires. It is unclear whether that would make the Calabim commoners see him as a threat, a savior, or perhaps both. The Octopus Overlords religion still contains some elements of devotion to Danalin, but it has been corrupted into something Danalin would surely despise.
The Fellowship of the Leaves fits the Calabim ok, as they can emphasize things like survival of the fittest and excuse their predation as part of the natural order of things (despite the supernatural origins of their secret ritual).
The Runes of Kilmorph does not fit the Calabim well. It emphasizes the value and rewards of honest hard labor in the way that a parasitic ruling class would not like. (It may be worth noting, however, that the Runes hero Arthendain was fighting in Prespur, trying to defend the commoners from Mary's plague, when the died and was brought back to serve Sucellus.)
The Empyrean is obviously the worst possible fit.
Vampires hate justice, but they do like imposing order. They wouldn't mind exploiting The Order state religion once the church has been corrupted. They still would not get along with truly devout men like Valin Phanuel, but I could see the wicked confessor Maraphene doing well among them.