These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Carpe Jugulum
Complete Monster: The young Count Magpyr and his family are this. The old Count, despite everyone insisting, isn't.
Fridge Horror: Greebo's ferocity and capacity for destruction have always been played for laughs. He's taken down bears, elks, and (once, by accident) ate a vampire because it was in bat form. However, under normal circumstances, Vampires are extremely hard to kill. Everyone who knows anything about vampires knows that. In this book alone, it takes complete incineration (normal fire, holy water, phoenix flame,) otherwise nothing less than decapitation followed by other traditional methods (such as the classic stake through the heart.) However, Greebo was able to kill a vampyre with simple, offscreen savagery. What did Greebo do to him? And if he could do that to a vampyre, what could he do to a human that pissed him off?
Greebo already killed a vampire once in Witches Abroad, though that was played for laughs.
Perhaps Greebo doesn't, in fact, 'know anything about vampires'. Being unaware that he can't kill the vampire, he just goes and does it, and the universe can't catch up in time to tell him that it is impossible.
It was a different kind of vampire, in bat-form, that he killed in Witches Abroad. Can't explain the case here though.
Being torn to pieces is apparently fatal for werewolves, despite their alleged invulnerability to anything but fire or silver. Angua admits she was in real mortal danger from wolves in The Fifth Elephant, and knew she'd have no chance against the Dog Guild in Men At Arms. It's possible that vampires have a similar weakness to being ripped apart: it's just not a tactic that human monster-hunters are equipped to utilize as easily as stakes or beheading, so they don't know about it.