A jury, chaired by Leslie S. Klinger (whose New Annotated Dracula belongs on everyone's shelf, in my opinion), will determine six nominees for the award and then select the overall winner.
I have my own guesses as to which vampire novel will be determined to have had "the greatest impact since publication of Dracula." My money is on Anne Rice's Interview with the Vampire, without which the modern vampire-as-tortured-soul novel, up to and including the entire subgenre of paranormal romance and the immensely popular Twilight books, would probably not exist. Of course, the jury will be fielding recommendations for possible nominees from the HWA membership, and since the membership has all but proven, year after year, that they tend not to read anything outside of works by brand-name authors and other HWA members, there's a distinct possibility the award will either go to Stephen King's Salem's Lot, which wouldn't be a bad choice, or some HWA member's small press vampire-as-serial-killer novel that no one outside the organization and the Shocklines message board has heard of, which would continue their long history of bestowing awards upon works that are an embarrassment.
But I digress. (Can you tell I still have some issues with the HWA?)
Anyway, I do think this Vampire Novel of the Century Award is a nifty idea, and I'm interested to see what wins. Share your guesses in the comments!