If anything, Buffyverse vampires (generally speaking) are the antithesis of regret.

Exactly! I can almost see the "hunger for life that's been damned to never be satisfied" part of it - James Marsters always talked about vampires as a metaphor for hunger, and he played Spike that way. But hunger is not the same thing as regret. Hunger is simply wanting something; regret is wishing you'd done something differently. Do vampires regret being turned? Rarely. Do they regret having to kill people in order to satisfy their hunger? Hardly. Not seeing the personification there.