I like the idea, it would add a new layer of strategy to approaching bosses like Vrykolaklaus, Survival Mode Nosferatu or even Lord of Darkness. Since Vampires often have a fair amount of evasion as well, maybe the Vampire Slayer could also have something to counteract this. Maybe not something as extreme as Ares Sword's Never Miss, but maybe a 50% chance that an attack goes through evasion, or else an on-attack effect against vampires in particular that offsets the damage lost by their evasion.
Okay, that's also a fantastic idea. You could also do an Immolation effect (Call it Holy Fire or whatever), and we could make the art something like a crucifix-shaped sword.