I have the Nikon VR-1 on the D700, and I can't imagine why I would switch to a Sigma version. Depends on what's important to you. You may not be able to find a new VR-1, but there are plenty of nice used ones for sale, and they hold their value.
If there was a big price difference (I don't think there is any)... and if I ran a photography school or if I had a staff of reporters, I may save some money with pallet of Sigma lenses, which are pretty darn good. But I would loose out when it came time to re-sell.
I don't have any CA on the fringes like I did with the 70-300.
If I turn off vignette control on the D700, you can notice a little bit at 200mm, but none with a TC... and with a good Nikkor TC it's always sharp edge to edge.
The VR-1 so super sharp in the center that it may seem a little soft in the corners to some, but I'm not convinced that it can be noticed except for tight corner crops from 200mm images, and only at f/2.8. The VR-2 is very sharp corner to corner, and so is the VR-1 with a Nikkor TCII such as the 1.4 or 1.7 or 2.0.
I have considered the VR-2, but the $600 difference isn't there. I can probably fit a VR-2 with a TC on the body in my gear bag, but the VR-1 is a little bit longer, so I can't.
For any aftermarket non-OEM lenses, always make sure that the particular serial number of the Sigma lens is compatible with the new cameras (if it was manufactured before a D7000 it will not work properly for example), and we're usually forced to buy used because they don't hold their value.
The Nikon 70-200 f/2.8 is my favorite lens. Beautiful pictures, much better than real life.