I have used both the Non VC and the VC versions (Canon fit) of this lens - both have been more than acceptably sharp.
I sold the Non VC version last year, so cannot now do a comparison, but from my experiences, the VC version is superior in that it has the VC image stabilisation system plus it is sharper...........
If you look at the review made by Digital Picture,
[click here] the VC version of the lens is sharper at 17mm - F2.8 than the more expensive Canon 17-55mm lens which everyone aspires to.
Whilst I do not doubt there may be soft copies of this lens like any other lens, it is my belief that the issue in the main may be attributed to the difficulties of shooting in poor lighting and wide open at F2.8.........
I certainly had some focussing issues with the Non VC version of the Tamron lens, sometimes it would struggle to achieve focus in poor light, but that is true of any lens in low lighting conditions where a lack of contrast will make focusing that much more difficult.
The best test is to use the lens in a real life situation rather than concentrating on the 100% crops seen in these reviews and tests, its a terrific walkabout lens that I am sure you will be very happy with.