"Damn!" Giles said. He crumpled the letter and made as if to pitch it into the bin, then changed his mind and flattened it out on the counter. "Dumbledore is demanding that I teach Defense against the Dark Arts this year."
Snape on the Hellmouth
Surely these crossovers have been done, though. Okay, google here we come.
Edit: Okay, found one of them, Harry Potter and the Vampire Slayer. Pretty much the first idea exactly! The author has only posted two chapters, so I dunno if she's going anywhere extra-cool with it yet.