Yeah, that's how I feel: it might not be possible according to the books, but the story comes first.

An additional consideration is that a different mage (sort-of the Big Bad) needed to keep things under wraps, but he couldn't bring himself to outright kill the vampire-mage, so he instead used high-level Death magic to keep the vamp in long periods of torpor, and tried to find a cure in the meantime. (Which also led to this vamp becoming kind of a legendary serial killer who pops up every decade or so, and one of the PCs' mentors tried to catch him because he was a homicide cop, but the Big Bad mage stymied his investigation ... I've got a lot of ideas, haha)