Just a thought.

Are you waiting until the VC 'locks on' before you press the shutter button?

I don't have your particular lens, but i do have the 24-70 and the 70-200 Tamrons with VC, and it is quite obvious in the viewfinder when the VC locks on.

I have my camera set to back button focusing so I use my thumb to focus and my index finger to activate the shutter button. I also almost exclusively use single point focus.