Seems you've made up your mind already, reading through your post. The Nikon 70-200mm VRII is an amazing lens, and every performance test and review testifies to it. Nikon pros will sing its praises for everything from portraits to sports. Of the "holy trinity" of zoom lenses it's the only one I have on my "buy this soon" list. It's also twice the price of the Sigma, so it does beg the question, "Is it twice as good?"
Without being able to shoot them side-by-side, the first place I always look when trying to decide between two lenses is
here. It allows you to compare similar test shots of two lenses at various apertures with a mouse-over. I look at all the factors, from sharpness to CA, note where they are similar and where they diverge. I avoid "safe" apertures and zoom areas and look at the critical areas of difference, which is usually at the extremes, and then ask myself how often that will impact me? Once I have that, I way those differences against differences in price, and potential resale value.
I've had a huge gap in my bag right between 85mm and 150mm for almost a year now (technically that closed a bit with the arrival of a 105 macro this week). The reason is that I am saving for the Nikon. No one I have ever spoken to has recommended a different zoom in that range for any reason other than budget, weight or expedience. Given that you have the Sigma, you need to decide if it's worth reinvesting. I can't tell you if your lack of sharpness is personal or lens related without seeing photos and EXIF data, but I can tell you that if it's personal then the new lens will only get you part way there.