The master of shadows seemed to manage to have quite a motley crew in marienburg. Given him as an example I think you'd need a vampire in a position of power they could exploit and you'd need quite a lot of empty space as the living might get a bit freaked out bumping into the undead.
Multiple groups would be tricky, as you say there'd need to be regular movement of people to cover up some of the more unsavoury goings on. That's why I think a port city would be a good option. The ghosts would have to side with a group as vampires can dominate lower forms of undead it would come down to who the more powerful vampire is I guess.
Mousillon, city of the damned. Port city, sea to the west and south, North gate leads out through swamps/marshes, east gate leads out through vast Necropoli, a by-product of plague and illness from the presence of the marshes. 2/3rds of the city are basically off limits, except for the bravest when the sun is high in the sky. The nobility (that are still human) keep themselves walled up in the temple/high districts, not far from the port.
Size is important, a small village can't realistically support multiple (potentially feuding) vampire factions and their undead minions/experiments.
Location in a region matters as well, if the locality is known to be cursed the local populace won't be as inclined to call the Witch Hunters at the first poltergeist sighting.
City build-up is important, a port city helps moving the bodies certainly but I feel a landlocked city could also work. What you need is plenty of abandoned areas. Sewers, alleyways, catacombs, crypts, nearby forests, abandoned warehouses and dungeons. Basically, places were the locals don't go after 10PM. In the Genevieve novels (which I would heartily recommend) there was a vampire bar in a shady part of Altdorf, where living humans took the places of beer kegs (nice and fresh blood) to pay of debts they had defaulted on.
That's just the basic infrastructure, now there are more factors to consider:
Integration in society, the vampires would need to control at least part of the flow of information (be it trough the local guard, local church or what have you) to cover up the occasional witch-lights or walking corpse. Either by dealing with the local rulers as equals, puppeteering them or supplanting them.
Some control over the vampires, this is important, if a vampire gets delusions of grandeur and decides he is going to raid the graveyards for 'supplies' you need something like a vampire sheriff to 'keep the peace' and quietely dispose of the silly vampire.
A code of conduct how the different factions conduct themselves, this doesn't have to be a formal set of rules (though it could be) but it really doesn't help if the Lahmian and von Carstein families are dealing with a dispute in a political meeting and the local Strigoi clan decide they want to throw down with Crypt Horrors.
Last but not least, I feel like there should be some benefit for a city to have a vampiric nobility, if the vampire are just parasites slowly draining the town there are only two likely outcomes: a) the town withers and dies, leaving the vampires with a city-sized graveyard or b) the mortals get fed up with years of oppression and terror and invite the Witch Hunters for a BBQ. This benefit could be anything, from having 'cheap indentured labour from a land far away' ("golly they do stink") to work the dangerous but highly profitable mines to having a potent guard of red-armoured knights to deal with local beastmen herd.
Your imagination really is the limit in these cases, I would recommend you google the game Vampire the Masquerade: Bloodlines as they have a lot of great angles you can use for inspiration.