It's not the vampire thing that bothers me. If done well, I'm pretty sure i'd even be a big fan of it!! It's more so the fact that this long arc (and it is EXTREMELY long), with already dire consequences (hunting dogs, the page, Decay of Angel, a literal sword that cuts through time and space, and the literal end of the world), had a new major plot point added completely out of left field. No build-up, no tension. Just, boom! suddenly vampires.
That's the issue I have with it. Eh. We'll see how it goes!!