Is the Nikon 105VR better than the Sigma 50-150 f2.8? Yes, certainly at 105mm. Significantly less good at the other focal lengths
You've got a 50mm f1.8 now, so you can see how that performs for portraits. Check the bokeh carefully, as the 5 blades tend to create swirly patterns rather than a smooth effect, particularly wide open. Try that, and the 55-200 at 105mm, and see if you those two lengths suit you. For example, I know that for my local zoo, 105mm is superb except for two situations - the big paddocks and the sea lions and penguins, The paddocks need more length, and the tanks need wide angle. I take the 105 and the 18-200VR, and use the 105VR whenever possible. Having had a look at the reviews, the 50-150 is EXTREMELY good, and had very nice bokeh too, so if you come to the conclusion that you need some flexibility, I think the Sigma would be a very sensible choice. I did notice a few comments about the focus being a bit off at times though(at 150mm I think). If you can live with the primes though, 105mm is a lovely and flattering focal length.
Here are a few examples of non macro Nikon 105VR shots. The lion and fennec fox are through glass incidentally, so aren't quite as sharp, and some are at a higher than ideal ISO. At 100 ISO, even wide open, this lens is pretty much as sharp as you can get.
400 ISO f3.2
400 ISO f4.5
400 ISO f4
200 ISO f5.6
100 ISO f3.2
100 ISO f2.8
and finally, since I can't resist the odd macro...
400 ISO f8
As for the 18-200VR (for psenior1), I'm pretty happy with mine. Ignore that zoom creep business - after reading about it on the forums I tested mine, and sure enough it does it. I would never have noticed it if people hadn't told me to worry about it
Kew temperate house, 18mm 400 ISO. 1/10th sec handheld...
It performs pretty close to the claimed 4 stops of VR.
The 18-200VR is going to Rome with us in a couple of weeks, so if you hang on, it'll get the most serious workout yet as a travel lens, and I should have the odd pic or two to show. If you're fancying a 70-300VR, I'd look at the 16-85VR as a companion though, as I find I don't use the long end much when I'm not shooting wildlife. I tend to travel with two lenses only - either the 18-200 and 105VR, or 18-200 and Tokina 12-24 f4, so for me the 18-200 is perfect, as it means I can carry chunky pro glass like the 105VR and still have most focal lengths covered.