The problem with this attack form is that it essentially bypasses the creature's hit points - the vampire in question could be of epic level, but a single critical hit wipes it out.
The real danger here, from my point of view, is that it begs the question, why can't this be done to every other creature the PCs encounter? After all, wouldn't a stake through the heart be just as effective on, say, an evil human cleric? It becomes somewhat hard to find a rationale to say why the PCs then can't use the same tactics.
Personally, I wouldn't allow it, simply because its more of a can of worms than its worth, not to mention it essentially means that any vampire ever encountered becomes far less challenging.