My first thought was along the lines of the sword holding the soul of a vampire, as you suggest. Perhaps long ago, the sword belonged to someone who was hunting down and killing the vampire's weaker minions. When he figured this out, the vampire made himself into a trap. Normally, the sword wouldn't hurt him at all. But he somehow made himself into a magic trap, so when he was struck by the sword he appeared to die, but went into it. He eventually corrupted the original wielder - who might be the assassin, or maybe the assassin later came after the now-corrupted vampire killer.
(The whole thing pivots on how much you're willing to make up the "somehow.")