Not that common. I think out of the 1200 or so d20 books (just a guess, used to know for sure here until they started taking non-d20 stuff, so I dunno the exact count), I think maybe 10 are on Vampires (including Ravenloft which I don't have).
The Bottled Imp book is quite good. And the author is a really nice guy.
I also thought the Green Ronin one, Fang & Fury was pretty decent.
The one from Bastion Press is okay, but is in part sort of a monster manual of vampires. There's also a PDF from some Scandanavia company which looks really good (Crothian reviewed it, I can't remember the name).