EDIT:
Wo! Hold on there. You've got a problem.
SRD said:
Size and Type: The creature’s type changes to outsider.
SRD said:
“Vampire” is an acquired template that can be added to any humanoid or monstrous humanoid creature (referred to hereafter as the base creature).
Half-Celestials can't become Vampires, RAW.
If you wanted to do it, anyway, then you'd need to follow Merak's advice, above. Specifically:
He'd lose the Con bonus from being a Half-Celestial, since he loses his Con score.
All of his Hit Dice would become d12s.
He'd have a net +7 improvement to Natural Armor bonus to AC (+1 HC, +6 Vamp)
Retain his Smite Evil ability
Retain his Daylight ability (Hah hah hah ...)
DR changes to 10 / Silver and Magic (Vamp overwrites HC)
He'll probably want to stay away from a lot of his HC spell-like abilities, as well. Holy Word? Summoning Celestials?
Yeah, might be a bad idea to use those ...
