This seems almost a ridiculous question, but since I can't find any rules regarding it...
Does cutting the head off a vampire for sure kill it? or if they had enough vitae could they regrow the head? or rejoin the head?
Page 7, "Truth and Lies"
VAMPIRES ARE IMMORTAL: Almost. There's very little that can harm us. Sun. Fire. Maybe a heart broken by a wooden stake, followed by losing your head to a hacksaw. Other than that, the years stretch out like a banquet.
Although the argument could be made that a head severed could be reattached like a limb (if Vitae were spent fast enough) decapitation does lead to Final Death.