Vampires are a bit crazy. Even in the original source material, they could recover from being decapitated, literally regenerating a new body...unless the decapitation was followed by destruction of the head and body.
HAH yes running water damage and could even kill them, so Techo-Wizards along the old Mexican boarder created magic powered water cannons, etc. against them. They could regenerate crazy fast and a stake through the heart only put them in 'torper' or whatever it was called. So yes super tough BUT with super weaknesses
In case it's not obvious, by giving the mage a chance to run, Dawn (and me) are trying to cut a difficult and lengthy battle short. Let's hope it works.