I am in the same situation of acquiring a good zoom for a D700. I have a Nikon 70-300 F4.5/5.6 VR, which works well. However, I am undecided on the options for improvement:
1. Get a 70-200m f2.8 VR lens for lower light use and short DoF (depth of field), but sacrifice higher end magnification. Could get a 1.7 teleconverter to get the reach, though.
2. Get the Nikon AF 80-400mm f4.5/5.6 ED VR, which has the reach but not the lower light / short DoF capability. This lens is reputed to be slower in focusing than newer designs.
3. Get the Sigma 120-400mm f/4.5-5.6 DG OS HSM, with the same comments on low light and DoF as 2 above. Has anyone used this lens?
I have the use of a Nikon 70-200 f2.8 VR for this week, and if the opportunity arises I can compare with the 70-300.
On the vignetting issue, here are some sample photos of the 70-200 f2.8 on a D700 FX. The second pic has had the contrast turned up in post processing to exaggerate the effect. The third is a 'normal' pic where vignetting is present, but does not intrude.
1. No processing.
2. High contrast
3. 'Normal' scene.
Edward