The bottom line:
Yes, VR will absolutely improve panning shots. But you also have to consider the type of panning which is being done. Example:
Panning with or without a monopod. If you're using a monopod, chances are that you're attempting to capture a fast moving object such as a vehicle or perhaps someone running. However, if you are panning and tracking an object with variable height and depth, then you likely won't be using a monopod (unless your lens is fairly wide and can encompass both depth and height for a variable object such as a ball flying through the air).
I did my research on the Sigma and Tamron 200's and did not like them for various reasons. The Sigma was a hunt machine. Hunt, hunt hunt, and more hunting with that lens. The Tamron didn't focus fast enough in dim light when compared to the Nikkor, and was also very noisy. My research wasn't rushed and I made a very solid decision, and am VERY happy with the results. The range, sharpness, bokeh and speed of the 70-300 is right on in my opinion. Sure it's not super fast (not talking focus, because that IS very fast) but the VR makes up for that fact, and its price is right on the money.
You also should take this last thing into consideration: Resale value. Take care of the Nikkor stuff, and you'll be able to trade or sell it with a lot more ease than a Tamron or Sigma. BTW, I"m not biased either, I absolutely adore my Sigma 17-70. The macro on that thing is stellar.
Doug