well, there are a bunch of different circumstances that might cause even Vanessa to use "playback"... there was speculation at some of the large performances she might have lip synced only because of the inability to mic her, run cables or what not in certain venues. By choice I would guess Vanessa never uses "playback" because she has commented on several occasions how much she hates it and the fact that other artists do it.
I know for performances like the Grammy's they have the artist come in and do a "live" version of the performance prior to the actual show just in case something happens (they get sick, nerves, whatever) they can play the live performance and lip sync to that or such...
-katia