I'm using the tamron 17-50 vc.
IQ is soso. Non-VC is definitely sharper at centre at f2.8. My VC gets reasonably sharp at center f4.0. But if you are a facebook shooter, it doesn't matter.
AF is reasonably accurate.
Only complain is the noise level, and the slow AF. AF is about 2x slower than the kit lens. You can youtube for this.
VC would help if you use auto ISO. For my body (550D), auto ISO minimum shutter is implicitly set at 1/30 (nothing I can do about it). Obviously if you are shooting at 50mm, it might cause blur especially if the body is not leaning against your face to take some weird angle shots.
Night time I usually use flash, so it doesn't matter whether VC or not since shutter at minimum 1/60. For cases where flash can't be used (landscape or some places like museum, underwater world etc) and you don't want to carry a tripod around, VC is a lifesaver.