Primitive Screwhead said:
This is an excellent RP hook and would add depth to the game experience, however the problem comes in on the mechanical side of things. If you simply apply the Vampire Template with all its kewl powers.. your talking a +8 LA. So either the character has to be so far below the rest of the group level wize that BAB and Saves all really suck, or so far above the group that the rest of the characters really suck.
On the other hand, you can also play up all the penalties for full vampirism. Can he enter Castle Ravenloft without Stradh's permission, for example? What about the lack of reflection and shadow, the need to sleep during the day, being damaged by sunlight, holy water, and garlic, being damaged whenever his RSoP companion turns other undead, being unable to cross running water or enter non-public dwellings without permission, and so forth? What about the need to drink blood? While some mythos allow for animal blood, I do not think D&D vampires are among them.
All that said, I suppose the idea of a the half-vampire template (in Libris Mortis). With an LA of +2 it is not crippling. Perhaps it could work this way:
A PC is attacked by a vampire. To become a vampire, they must be drained at least 2 levels and have all of their Con drained via blood loss. They then come back as a half-vampire with the Special Attack "Blood Drain." This ensures a need to drink blood. After enough time (or, perhaps, enough blood drained) they become a full vampire - and potentially an NPC.
As they were drained two levels before becoming a half-vampire their power level is about where it was before - so long as there is no quick and easy way to restore their levels. Or, if there is, perhaps this character started a level below the other PCs. Now he is equivalent to a level above - not too overpowering, considering.
The Half-Vampire template does not grant everything a vampire has (such as spider climb, gaseous form, etc), but it does grant some energy resistance (cold and electricity 5), some DR (5 / silver or magic), and some fast healing (2). The Blood Dependency that comes with the bite attack is the real issue, but otherwise the character can walk in sunlight, is not harmed by turning, and so forth.
In becoming a half-vampire perhaps he has merely taken the first step (like Mina in Dracula), and has not yet (like Lucy) become a Vampire Spawn.
The Half-Vampire template is on page 107 in Libris Mortis. I strongly suggest using Blood Drain as an automatic Special Attack for those taking this template, as otherwise the character is gaining multiple advantages with no disadvantages. (Blood Dependency - the only penalty in this template - is not granted unless 'blood drain' is selected among the three choices for the special attack. The other two special attacks are charm gaze and children of the night.)