... sell away my tamron 28-75 and buy a tamron 17-50 VC? I am attracted by the VC, and also can cover events better since 17mm on a 1.5 crop can cover wide angle. My trade off is the reach for beautiful portrait at 75mm.
You seem to be relatively experienced. So you probably have an idea of which focal length range is better suited for your event shooting style already, right?
Personally, I don't think VC/IS is that much of an issue for events, cause at the slow shutter speeds that you see its benefits, you're going to have issues with motion blur (subject movement). I rely more on flash than fast lenses, so I'd prefer a 17-70 or similar. If you prefer a constant 2.8 aperture, then your decision is primarily whether you'd miss the wide end or the tele end more, and if either is indispensable. I'd stick with the 28-75 myself, but that's just me
Personally, I'd stick with the 28-75. The wider end like 17mm don't attract me that much for event photos as they usually end up being 'this was this'; 'so many ppl were there'; 'just get everyone into the photo' type shots. In short, typically very messy photos, lack of focus on the people involved, small and pushed back subjects. Not my preference anyway.
Of course these 'get everything in' shots may be what the layman wants from a photographer, so when it comes to that I'd just step back at 28mm or switch to a wider lens.