Not to mention that she has also stated that humans can't kill vampires, even stating that a bomb wouldn't be enough.
We could take that to mean that most humans would be out of their depth trying to kill a vampire, and that no conventional weapons could destroy them.
Magic is barely touched upon in Twilight, so I think the Vampire Killer would still be effective. Also, Belmonts kind of push the "human" definition a little further than Meyer did.
I guess what I'm saying is that Edward could (and probably would) be killed by a Belmont of Simon or Julius' caliber. Especially since Meyer states that his speed is his greatest asset, and he's actually physically weaker than most of his family. He'd probably just pussy out and try to talk things over though. And then Belmont whips him GOOD.
Now... EMMETT on the other hand...
DUN DUN DUUUUUUUUUUN.