Vampires don't need blood to survive; they need magic. It's just convenient that the blood of most races contains magic and is thus the easiest way to sustain them (for example, a lot of Necrarchs don't feed any more; they siphon magic out of the air into their bodies).
Sorry but this is mostly wrong. The legacy of Neferata, the "Elixir of Life", imbued humans, whom it was specifically designed for, with immortality. This came with a price - the need to feed on fresh blood.
Necrarchs are scrawny because they dont feed as often as other vamp bloodlines, they are too busy studying necromancy, and thus become weaker and more frail. Strigoi's fed on tainted or dead bodies and the nature of the decomposing blood changed their nature over time. The other bloodlines have similar feeding preferences that have shaped their background for centuries.
Also, I believe that the problem of never seeing vamps from other races can again be explained by the Elixer of Life. It was created for humans, and while they may not understand genetics in the conventional sense, it must have been alchemically and magically "attuned" to human physiology for it to have the desired effect.
All vampires that exist currently have been "sired" by another, older vampire. The Elixer is lost, so the successive generations of vampires have become less and less powerful.
I can only make an educated guess here, but its my opinion that the genetic requirement of the elixer that now flows within the current generation of vampires may not be quite as specific as it once was, so there may occasionally be the odd "exception" to the humans only vampire. They would be extremely rare, however, and the snobbish and picky taste of vampires in general might also go some way to explain the lack of any other vampire types.
"I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing."Visit my Tower Terrain Plog here.
"But," says Man, "the Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED."
"Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic.