MI found a ruling that says:
"An ally that plays a card "as a vampire" is treated "as a vampire" for all puposes of the resolution of the play of that card including any continuing effects, but not for effects the card generates for being in play."
This suggests that the ally lives long enough to damage the opponent.
What often causes confusion here is that people often want to treat these effects as turning the ally into a vampire for the duration of the card. (Similarly, if you can play a card "as a Brujah" like Mata Hari can.) But that isn't what happens.
Decapitate sees you as a vampire with inferior potence and X amount of blood, for the purposes of playing it and resolving it and, if it has a lingering effect, the lingering effect. It sees you as being a vampire, with the requisite level of Potence, and enough blood to pay for the card, and that sort of stuff.
Importantly,
nothing else in the game sees you like that. You are still an ally. You can still be targetted by effects that affect allies (e.g. superior Eyes of the Serpent, inferior Entombment, the extra damage from Coagulate Blood) - yes, even when you're playing Undead Strength as a vampire, your opponent can hit you with Coagulate Blood as the ally you really are. You can't use the special text on your opponent's Meat Hook: "At close range, a vampire with Potence [pot] may destroy this weapon as a strike with first strike that costs 1 blood" because you aren't a vampire with Potence, except for the cards that see you as one because you're using your special ability (like Decapitate or Undead Strength or whatever).
And since you're still an ally, the normal game rules on allies kick in. No life? Burn horribly.
when does an ally burn in combat when playing cards "as a vampire"
