Well, okay, I'm gonna step away from game balance now because I haven't played a fraction of the games most people here have played and as such my insight into balance is probably going to be lacking.
I'll approach this from an angle I know: from the storytelling angle. You can still say that all my arguments here are irrelevant because game balance demands that Vampires aren't undead, and that's cool, but hear me out:
When you create a fantasy setting, you don't build in a vacuum. Not only is there a huge body of literature written in the genre, but fantasy is also peculiar in that it is based to a high degree on the first kind of storytelling there ever was: myth. Of course you can do things differently; the most interesting fantasy books are those that take a central concept and turn it upside down. Point in case: John M. Ford's "The Last Hot Time", a gangster story in a Chicago in which the gates to Faerie had opened again. His elves torture humans to gain magical power, shoot at each other with tommy guns and are generally ****ing sinister. This works because we don't expect elves to be that way.
To turn a concept upside down works very well. To change a concept just a little bit, not so much.
It's like in music. If you construct your chords out of notes that are distant from each other, if you set your main melody to let's say a counterpoint melody, that tends to work. If you just move off by a half-note, though, you're gonna get some ugly disharmonies. People expect vampires to be undead. This doesn't need to mean that people think in a mythopoeic microbiological way; it may simple mean that they apply some of the most salient features of the undead (no natural aging, no death through starvation (at least not NORMAL starvation), no soul) to vampires. If you just take this aspect of vampirism away and if there are no consequences, you have a messy 2nd chord. It'll be easier to make your readers/players accept that YOUR vampires **** flowers and live on trees than to just slip in a little "oh and btw my vampires are actually people, but they still drink blood and they're still sinister etc". You know that truism about people being more prepared to believe the impossible than the improbable? That's where it comes from.
So in short: a little change (especially one which doesn't have further consequences in your world) is a dirty, unclear story-telling mechanism. (rereading this post I feel the need to explain my use of "dirty" here; I looked for a better adjective but couldn't find one. What I mean is that there is CLEAN story-telling, where the concepts fit together and everything feels just right, and there's the kind where there must be sand between the wheels and cogs because the entire machine makes unhappy noises and you just feel that something's not QUITE right; I'm a little bit too philosophically inclined, and in these last 15 years or so that I've been creating fiction in one form or another (I'm explicitly counting the years in which I "only" created content for my rpgs) I've been thinking about these strange feelings a lot. WHY does a certain plot work, why does another not work? I don't think I've quite arrived at any final answers, but these approaches I have right now seem to make at least some kind of sense to me)
However you can of course say that they need to be not-undead because of game balance, OR you can say that the consequences of them being not-undead are still forthcoming. Both are good
--edit: haha, I **** asterisks ;P