I had the chance to shoot with the Tamron 17-50 f2.8 non VC and by no means was I impressed. It was as sharp as a marble wide open. I did a focusing test on it and no matter what setting I did for fine tune focus on my D7100, it always seemed to front focus about 7mm when shooting wide open. Once I got down to f4 it began to correctly focus and the images looked sharper. I ended up buying a sigma based on a recommendation given by someone on the forum and I am impressed by the quality it produces. See if you can test the Tamron out for a few days before purchasing.
Search online for any front and back focusing problems with the lens. I had to sift through a lot of garbage, but what I did find was that some lenses have focusing issues (Nikon, Canon, Pentax, etc) and had to be sent in to Tamron to have the firmware updated and then checked to make sure the calibration was correct. If not under warranty you're looking at a minimum of $130 to have them do the update.
Hope this info helps!