Yelchin should play the vampire.
Pretty much any competent under eighteen actor could do the juvenile lead. The kid from Breaking Bad comes to mind, he does a terrific job as a cerebral palsy victim. R.J. Mitte?
Screw the Peter Vincent role. That kind of Vincent Price plumminess is hard to come by, outdated and not intrinsically necessary.
The real difficulty, which I'm surprised you guys didn't notice, is casting Evil Ed.