I didn't feel that my first one was within spec with the VC (I would get the jumping and inconsistent VC performance).
Since I'm most probably going to buy the Tamron, too: could you please write how you tested the lens for "proper" vc performance and came to the conclusion to replace it?
That'd be really most helpful for us budget Tamron customers :-) ... the good thing about Tamron for sure is that they do free lens-body calibrations and have the 6y warranty.
First, I had an idea how Tamron's VC system should work, having using a couple of copies of the 70-300 VC as well as the 18-270 at one point (and I don't think that the 24-70VC system is quite as comparatively effective as the 70-300 - it is stunningly good). I didn't feel like the first copy was working to spec, so I ran a few tests:
I approached it in two ways: I did a series of images with a bookshelf filled with books (very fine details) and shot multiple series of photos at different focal lengths with VC on/off.
I did a similar series for a reference point with the 24-105L, 70-200L IS, and the Tamron 70-300 VC.
It became quickly apparent that the Tamron system was not consistently working by comparison. My field tests revealed the same issue. I was also getting some of the jumping in the viewfinder as the VC engaged, and sometimes an apparent delay before VC was engaging (which wouldn't suit for my work - I do a lot of event work and weddings where you don't have time to wait for the system to engage).
I had no problem exchanging with the online retailer I bought from, and I have been thrilled with my second copy in every way.