Nebulous
Legend
You know, you would think that more vampires would wear armor. Or at least breastplates. Have the BBEG be a Vampire Paladin or something, and rule that they can't get to his heart for as long as he's wearing his armor.
I agree, but in the abstract AC of D&D, it wouldn't make much difference. Unless, as you said, the DM made a special ruling. Or hell, keep his heart in a phylactery like a lich does!