Maybe this is a special case?
If I remember correctly, the other vampire said something about practically being a different person, meaning that he probably wasn't literally a different person, since he had no reason to lie.
You'd think not, though his assertion that "who he was centuries ago" is different than who he is now, as though he'll lose his memories or personality or something, might counteract that.
Also in favor of special-case is that NegaDurkon didn't seem very good at being a person, whereas Snakey was. If that was the same kind of negative energy plane spirit as NegaDurkon, it's apparently one that had become either
very accustomed to its new shell or was
incredibly good at faking it for murderously long periods of time.
On the other hand, we have no idea
why he'd be different, and there's Belkar's speech about how people don't change instantly. The former I guess could have something to do with the sunlight protection spell; that does seem like the kind of thing any spellcasting vampire would want and eventually be able to get, so maybe it's only possible through divine intervention which has some interesting connotations for vampiric spawn. I wouldn't tend to think he'd have done that knowingly or on purpose, owing to the previous issues with being pretty good at acting like a person and seeming to like Durkon a lot personally.
Belkar's speech troubles me, though, because it feels a bit like an author rant to me. It wouldn't surprise me if that was one of the original or major reasons why he felt the need to split off Person and VampirePerson into two entirely separate entities. Though at this point it might be worth mentioning that I don't like or respect Rich much, so maybe that's just not giving him the benefit of the doubt.