Reading through the XML, I find Decius talks a lot about having been king of many nations, of being an exile, of having seen a lot, et cetera. I get the impression he's a skilled and experienced leader with the ability to charm, backstab, and earn his way to the top of any nation he finds. Sort of a professional travelling monarch. Why the Bannor, Malakim, and (most bizzarely) Calabim are his chosen "clients" is beyond me.
I suppose that all three are relatively fluid nations. The Malakim are enlightened and welcoming (EDIT: former) nomads. He could quickly prove his skills there, and earn his way into the trust of Varn Gosam himself. The Bannor value martial skill above all else; even a stranger, if he proved to be noble, honorable, and capable, could become a great and important figure. "But what about the Calabim? Aren't they a rigid caste system?" Aye, but the smartest and the fastest can join the Moroi (or however it's spelled) and, if they survive long enough to recieve the "gift," climb the ladder to the top. If Decius is as expirienced in dirty politics and staying alive as I expect, he'd get bit mighty fast, perhaps by trickery, and then start knocking off his fellow bloodsuckers at a rate that would make even the best vampire slayers gape.
But this leaves a few questions. I'll disregard the Kurioates and Grigori; there are many lore reasons the Decius I'm imagining would lead them, and others that argue he would not, but the gameplay problems are obvious. "What about the Doviello; if this theoretical Decius is such a nomadic tough guy, surely he'd rise to the top of Doviello society just as easily as the Bannor?" This does put a hole in my theory, aye. But look again at the chosen three. Two are definetly feudal nations; the cruel Calabim have clearcut hierarchy of vampires, Moroi, and serfs. The Bannor use a combination of feudal and military hierarchy. I know little of the Malakim lore, however. I'm choosing to assume, perhaps incorrectly, they have a hierarchical and/or feudal government as well. The Doviello tribes are a whole different beast. Can you imagine attempting to arrange them into feudal states with agricultural serfs? It'd be like trying to train wolves to work as sheep dogs. "OK, but I'm sure the Amurites wouldn't mind working for a professional monarch. Agricultural systems would fit him fine." Aye, but Decius hasn't got a drop of that old-time voodoo in him. This has both gameplay and lore implications. What would the Amurites be without being Arcane leaders? This is my answer to both lore and gameplay questions; both Amurite leaders are capable mages as well as politicians. Decius does not seem to have any magical skill whatsoever; I doubt a council of mages would like taking orders from a guy who can't even pull a rabbit out of a hat. "How about them Balseraphs? They're definetely a feudal system!" I can hardly see Decius leading them for long, however, without either fleeing in terror and/or frusteration, or getting killed.
Of course, this is my pet theory. I might be partially right, nearly right, on the money, or (most likely) a million miles away.
EDIT: I'm going to check Decius' preferred civics, see if that gives me any fuel or kills my mad rantings outright.