Losing Constitution until you hit zero typically results in just plain regular death, not undeath in general or vampirism in particular. I presume you mean losing Constitution specifically through a vampire draining the character.
The ability that does that is the vampire’s create spawn:
If the vampire instead drains the victim’s Constitution to 0 or lower, the victim returns as a spawn if it had 4 or less HD and as a vampire if it had 5 or more HD. In either case, the new vampire or spawn is under the command of the vampire that created it and remains enslaved until its master’s destruction. At any given time a vampire may have enslaved spawn totaling no more than twice its own Hit Dice; any spawn it creates that would exceed this limit are created as free-willed vampires or vampire spawn. A vampire that is enslaved may create and enslave spawn of its own, so a master vampire can control a number of lesser vampires in this fashion. A vampire may voluntarily free an enslaved spawn in order to enslave a new spawn, but once freed, a vampire or vampire spawn cannot be enslaved again.
In this case, I presume the character in question has 5 or more HD, since that is the requirement to get a vampire out of it. Note that unless the vampire is biting off more than they can chew,1 the vampire will have complete control over the vampirized character—that may be relevant to you. Or it might not, depending on what you’re doing.
Anyway, the effect is that “the victim returns as a [...] vampire.” What does that specifically mean? It means they gain the vampire template themselves:
“Vampire” is an acquired template that can be added to any humanoid or monstrous humanoid creature (referred to hereafter as the base creature).
A vampire uses all the base creature’s statistics and special abilities except as noted here.
And one of the things noted “here” is that
Increase from the base creature as follows: Str +6, Dex +4, Int +2, Wis +2, Cha +4. As an undead creature, a vampire has no Constitution score.
Having “no” Constitution score is different from having “0” Constitution! This is the key to understanding how this works. Constitution becomes a nonability for the new vampire. A nonability is defined as so:
Some creatures lack certain ability scores. These creatures do not have an ability score of 0—they lack the ability altogether.
This has a general effect for any nonability:
The modifier for a nonability is +0.
as well as specific effects for each ability score, which for Constitution is:
Any living creature has at least 1 point of Constitution. A creature with no Constitution has no body or no metabolism. It is immune to any effect that requires a Fortitude save unless the effect works on objects or is harmless. The creature is also immune to ability damage, ability drain, and energy drain, and automatically fails Constitution checks. A creature with no Constitution cannot tire and thus can run indefinitely without tiring (unless the creature’s description says it cannot run).
Also, since vampire says “The creature’s type changes to undead,” and the undead type says such a creature “Uses its Charisma modifier for Concentration checks,” the +0 Constitution nonability modifier would be replaced with the character’s Charisma modifier on those checks.
So the character loses their Constitution modifier entirely, treating its bonus as 0 when relevant. Any Constitution they had before is gone, but so is the −5 penalty they had at 1 Constitution just before they were fully drained.
A word of caution
If this is a player character, please know that the vampire template is crippling. It applies a Level Adjustment of +8, which means the character is now supposed to count as being 8 levels higher than they otherwise would be. The template is not even remotely close to being worth 8 levels. When this happens in play, they receive a fairly large benefit on the spot, but every time their allies level up, they won’t, because they already “have” that level. They’ll gain XP, but the XP they need to level will be whatever is required for the level 9 above their own, rather than 1. Soon enough, the levels their allies are getting that they aren’t is going to leave them in the dust.
Moreover, vampire comes with all kinds of notorious drawbacks and difficulties, that make it extremely difficult to work into an ongoing campaign. Expect that the vampire’s new difficulties will become a major focus of the campaign—which may be a problem for the party in terms of spotlight. Or, if the party neglects those difficulties, expect the vampire to struggle to even keep up or participate—perhaps the opposite problem for spotlight. Either could be overcome, but it’s going to require quite a bit of effort for all involved—great if that’s what you want, but not so great if you wanted to get on with your ongoing quest.
So I strongly urge you to reconsider this. The rules for vampirism are extremely problematic and difficult, and this has a fairly high chance of having a rather-negative impact on your game.
- Honestly was unintentional until I was half-through writing it. Couldn’t stop there.