D800 with 24-70mm and VR question.

VR is basically 2 stops...if you can use it.  In other words, if you can live with the slower shutter speed not causing motion blur with your subject, you gain 2 stops of light with VR.  For example, if your exposure at 70mm was 1/80, F2.8 and ISO 6400, VR would allow you to use 1/20, F2.8 and ISO 1600...a visible improvement in IQ.  VR also acts as shake protection at higher shutter speeds, when your technique isn't perfect.

The 24-120 F4 VR is a decent lens, but of course you don't get F2.8 and it won't be as sharp as the 24-70 F2.8, at F4.

There is also Tamron's 24-70 VC to consider.  It has gotten a lot of positive reviews.  I've got one and it is very sharp wide open.