Flavor-wise EVERYTHING can fit. Cleric turned Vampire. Vampire being Rebuked into becoming a Cleric. Vampire Blackguard hardcore born again Christian Pelorite with a Helm of Opposite Alignment. Half-Celestial turned Vampire turned Pelorite over a chick with a sweet ass.
The question is, how do we squeeze cheese out of it?
Believe it or not, even at stupid LA, Cleric is still one of the damn best career choices for a freaking vampire.
Why? Because it's got some of the best options, and because its base stats are made of stupid.
For starters: dude is undead, so he doesn't need to worry about Fort saves, crits or even his HP. What he needs to worry about is turn resistance, how to cope with his bloodlust and how to keep himself healed.
Assuming we don't need to keep a Helm of Opposite Alignment on this guy, a Hat of Disguise solves his first problem (after all, if a Cleric doesn't see him as an undead, he has no reason to turn him. And since the guy's already a Cleric he has no reason for suspicion, right?). The bloodlust problem is solved by keeping a sheep or more handy. Keeping himself healed might actually be tougher, given that Pelor kinda frowns on the whole flict-inay series of spells. Assuming Tomb-Tainted Soul is an option to heal yourself, that takes care of that...
Build-wise, you might as well go straight Cleric. You have precious little feats to spend on almost anything else, so you gotta make them count. Obviously, self-buffs are the way to go since actual undead-killer spells can backfire all over you.