Hmmm. To take it into the realm of actual speculation.... Okay: Yes, vampires regenerate in the stories in most cultures. And, if you go back to sources before Stoker's canonization of the theme, they tended (at least in Eastern European sources) to sleep in coffins filled with the fresh blood of their victims, and their flesh was incorruptible. (In fact, that's part of where the idea came from; according to the Russian and Greek Orthodox belief, it isn't the flesh of saints, but of the particularly damned that does not return to God's earth. When bodies were found upon being dug up to look as fresh as the day they were buried, they were believed to have a pact with the devil, and that they walked and fed on the blood of the living... especially when there were cases of actual pernicious anemia -- not at all uncommon with the poor diet in such districts at times -- or various other "blood diseases" afoot.)
As for zombies: In some of the literature on the subject, zombies don't eat at all. In others, generally the more authoritative, they tend to eat, but are often fed a porride (very much like gruel), as they have no sense of taste. Also, in real cases of so-called zombification, their flesh does not rot until they meet a second and final death. There have been cases of "zombies" which were used as slave-labor for literally decades (see, e.g., Seabrook's "Dead Men Working in the Cane Fields"); and one particularly famous case, Narcisse, was known in his native village for 20-30 years, recognized by his family, and friends, and his death was documented by the local hospital, as was his interment. There is some dispute as to whether this was actually the same man, or a man who had a striking resemblance, but it's never truly been settled. Was he actually dead, or had he been the victim of a "sorcerous" plot, using the tetrodotoxin that has sometimes been claimed to be linked to cases of zombification (again, this is in dispute, but it's damned difficult to get the facts about such obscure parts of pharmaceutical anthropology).
Now... if a zombie can eat, and a vampire regenerates, then the digestive system must be working at least at a sluggish level in both, and new cells must be generated. So I'd think that a zombie-vampire, feasting on the blood of the living, would indeed continue to exist in that state almost indefinitely. The difference is that such a creature might not be subject to the "second death" that a "normal" zombie is... and yet one of the most notable things about a zombie is their poor mental capacity (as the people who have been labeled such are generally somewhat brain damaged from being buried in a comatose state, and suffering oxygen deprivation enough to impair some of the mental faculties -- for the fictional zombies, it's because they are reanimated corpses, but with some faint semblance of life, such as sluggish digestion and elimination, extremely shallow breathing, the ability to take simple commands, etc.).
I'll give it some more thought, and see if I can find more on it in my books on the subject... but I think the combination is a unique one so far; but not by any means impossible. As you managed to convince me with the sources you cited in the "Dead Werewolf Becoming a Vampire" thread back shortly after I joined on here, Nesa, it may sound like an unlikely or even impossible combination, but when one looks at the genuine (not Hollywood) sources, it's not really all that far-fetched.
