Vampires get powerful abilities, and an evil PC should be able to choose that path
So munchkinism should be encouraged?
Put me in the camp that says that vampires should be monsters, only. You want to play a vampire, do it in a White Wolf game. But in D&D, there's way to much payoff in comparison to the penalties. If becoming a Vampire becomes a playable option, then you'll have PC's hoping they'll run into vampires, so they can be turned into one, and get a major boost to their abilities.
Nor are they comparable to Lycanthropes. You risk contracting Lycanthropy just by being bitten by a Lycanthrope, which occurs easily enough in a combat. Vampires, however, have to go through some convoluted rigamarole to create a vampire, and that sort of thing doesn't actually occur during combat. In some cases they simply have to drain a person of every last drop of blood, killing them. And no vampire is gonna stand there just draining one person while the rest of the party is wailing away on him with Magic Missiles and Flaming Longswords. And that's the easiest way. The other way is the vampire has to drain them to near death, and then force the person to drink his vampiric blood. That's something which DEFINITELY doesn't occur in combat. Who knows which method WoTC will choose for the 4E vampire, but no matter which, it makes no sense for someone to become a vampire just from fighting a vampire.
I wouldn't mind seeing a Dhampir. That makes more sense, as it's a race which you're born into. It's much better to have a character like that, which is balanced with the rest of the party from the start, then have a character who's balanced with the rest of the party, and then he becomes a vampire, and suddenly he eclipses every other party member. That's crap. And if a PC who becomes a vampire doesn't eclipse other party members, well, that's crap, too. Vampires aren't a prestige class. They should be preternaturally strong and fast from the moment they become vampires. And their power should advance by age, not level.
So yeah, when a PC becomes a vampire, he should become an NPC. Best to keep vampires as they should be, inhuman monsters who relish in feeding on humanoids. The last thing I want to see is a plague of moody, goth vampires who're trying to fight their inherently dark nature but occasionally succumb to their thirst (Usually when they need a power-up before they fight the Big Bad).