["It's going to take a little time to get it done. Witches who can do the spell don't grow on trees." Caroline then glances down at the bag. "And I think it's a little late for you to be negotiating."

You're already a vampire, Stiles.

Elena shrugs.]

It can't. Unless you happen to be in possession of a one-of-a-kind vampire cure. But once you take it, vampire blood doesn't work on you anymore. So you wind up in situations like this one.