It's nice to see vampires getting a little love. As a common, it's not that bad, especially considering the new Undying mechanic.
Until the entire Dark Ascension set is spoiled, I'm going to consider any creature with Undying as completely playable in limited and possibly any casual budget deck. It's a built in 2-for-1 and those are always great. It's appropriately costed (for a common) and it's a vampire. There isn't too much to complain about in this one.
As it stands right now, vampire tibal decks want to be aggressive, put on the pressure, and keep the pressure there as it steps on your throat and laughs. This vampire fits that deck quite nicely. As far as casual Legacy decks are concerned, I can't really see a place for the stalker outside expanded vampire decks. After all of Dark Ascension is spoiled, I'll bet there will be at least 1 or 2 other, better alternatives than this Undying common vampire.