What I like about your idea is the neato storytelling shortcut you have right there:
Lots of fiction/legends establish Vampires as a force of primal evil and great power. You don't need to build them up as bada**es. Anything that can hunt them down and wipe them out is obviously pretty hardcore and not to be messed with.
If you tried to establish a new critter as hardcore nasty, and then have your new race wipe them out, you'd have to waste a lot of time and energy establishing their hardcore status, just so you could then wipe them out to establish the harder-core-than-thou nature of the new guys.
Nice.
I think you pressed the wrong button (happens to everyone.)
Yeah, that like, totally happened.
Man, I'm bummed. But you get the idea.