One of the first thoughts I had was that vampires, while certainly extremely powerful, are very limited as well, at least in their classical iteration (which happens to be also the D&D version--it bears a very close resemblance to the classical Bram Stoker picture of Dracula and other vampires.) So I needed to come up with some ways around those limitations, either through social or magical (least preferred) means, or through having vampires be a bit different so that they're not so limited after all. This primarily manifested as the idea that there is an entire caste of regular "mortals" that serve and do the will of the vampires when the vampires are not about, either because it's in the daylight, or for whatever other reason. These mortal followers are called Untouchables, because in return for their service, they and their immediate families are protected by law from the depredations of the vampires when they need to feed.
The second thought I had was that variety in vampires is fun. Although I never really played either game, I'm familiar with the vampire clans of White Wolf's old Vampire: The Masquerade roleplaying game, and the vampire houses of Warhammer Fantasy Battles Vampire Counts armies. Since, I'd also read the Dresden Files books, which have four "Courts" of vampire, and each court is a completely different kind of monster (only three are actually detailed so far, although I expect that eventually Butcher will get around to having a Jade Court antagonist.) Despite the fact that the standard D&D rules do not really have vampire "varieties" like this, I want to incorporate them into Tarush Noptii, either by having additional templates to layer onto the current vampire template, or by creating different versions of the vampire template itself. Or, perhaps more easily, by taking some other humanoid life-stealing monster from the existing rulebooks and having them be different varieties of vampire. I even made a list of vampire houses, mostly by taking the word of vampire in a few different languages and adopting it (Ubyr, Vyrko, Vetala, Nosferatu) or by taking extremely famous vampire names and adapting them (Orlock, Dracul) or by taking the name of somewhat vampire-like other folkloric creatures and adapting that instead (Strix, Rusalka). What exactly these houses are like and how they differ still needs to be determined.
The source of all vampirism is a crash-landed death-god of some kind who lives deep in a crater in Tarush Noptii. Buried under tons or bedrock, locked by iron gates and chains, and warded by among the most powerful and mind-blasting sorcery known to mortals of a bygone age, this powerful god would still have awoken and destroyed the world ages ago but for the sacrifices of twenty heroes, who volunteered in the mad days of the god's fall eons ago, to absorb some of his power and hold it in, separated from the god itself, so that it would remain too weak and rise or wake fully. Of course, absorbing divine essence from a god of death means becoming one of the primogenitor vampires, the very first of that cursed state to exist. Driven mad by the power locked within their bodies, these nearly god-like vampires roam the vaults below the capital city of Vészok still, locked away with the death-god. Before they were completely sequestered away, though, some of them managed to attack mortals nearby, and from them spread the curse of vampirism. In this way, vampires, while tragic, are also at least somewhat heroic; their cursed state exists to protect the world from extinction at the hands of death itself. Of course this was many centuries, possibly even millenia ago. Few remember this, even amongst the vampires themselves, which opens up the possibility of the foolish and short-sighted digging deeper than they should and meddling in affairs over their heads. This is important to me because I want to avoid the easy and even cliched response of, "Let's overthrow the vampire tyranny!" as an immediate response to the situation. The players will have to live with the reality of Tarush Noptii as is, at least for the time being.
The other side effect of this death-god buried in the ground under Vészok is that it is always night over the city. The sun never rises. This is, of course, astronomically improbable, but this effect is a representation of the subconscious of a sleeping god, so it happens anyway. Geographically this isn't a wide-spread phenomenon--the entire diameter of this constant night area is only a few miles across--but there are hints that it is growing slowly year over year. Many of the vampires, who have forgotten their heritage, see this as a good thing for them, and are working to see if they can hasten the spread of the night.