the digitalpicture has done some vignetting tests on the new Tamron 24-70 VC. Looks like he has a really good copy because I'm not seeing much at all in his results.
I guess this proves without any doubt that the vignetting on the Tamron 24-70 VC is actually better than the Canon 24-70L mk1. That makes me feel much better about getting the Tamron 24-70 VC someday as an upgrade from my Tamron 28-75.
Perhaps the reason the initial reports made such a stink about vignetting is that the Tamron doesn't fall off as gradually as the Canon mk1, so the vignetting may seem more noticeable.