I also have the Nikon 105VR too, and while it's a great lens, it's not an uber fantastic macro lens, and I think the Sigma 150mm is a better lens for macro work(sharper).
The Tammy will match the Nikon in just about every aspect, both bokeh(as a portrait lens) and sharpness.
As Lani said, the 105VR comes in handy when doing portraits as the VR allows you to get more keepers at slightly slower shutter speeds(such as 1/40-1/60s), where subject movement hasn't caused problems.
I also had to decide between a Tammy 90 and Nikon 105VR and went for the 105VR due to the VR feature too.
Can't say I have any macro shots taken where the VR has helped at all.. and that means real macro images at 1:2 or 1:1 ... not just closely focused shots. For chasing insects at pretty close range the VR will come in handy, but I doubt it'll help anyone for proper macro closeups.
The term macro is becoming a pretty loose interpretation of something that is clearly defined too nowadays, so when you say you want to do macro images, do you mean very closely focused images of stuff, or real macro where the subject is almost touching the front lens element?
if you want to do a lot of 1:1 macro, then forget the VR feature, and factor in a very sturdy tripod and head into the price(do you have a tripod, and is it very sturdy?)
Things about each lens that I've noticed along the way:
Tamron 90, great lens, would buy one in an instant, but the annoying focus system lets it down. in every other aspect this is the best lens overall in a value for money sense.
Nikon 105VR: size.. BIG!(for a 105mm lens) but this never bothers me. the AF-S was a clincher for me. this lens has trouble auto focusing sometimes, even at 2m focused distances or more. it winds in and then out and can't decide what it wants to do. BUT!! the AF-S is the key here, it allows you to twist the focus ring in real time and then the lens focuses perfectly. VR is great to have when using VR is appropriate. If you can afford it and justify the expense, this is the lens to have over the long run.
Sigma 150mm : Has HSM(same as AF-S) and from my brief play, focused very nicely. Images look sharper to me, than the Nikon at close focused range. Most handy feature is the tripod collar. When doing proper macro stuff, you need to understand the importance of this simple addition. Model I played with didn't have Sigma's OS variant of Nikon's VR feature, but I think the newest version may have. Sigma is cheaper than the Nikon, and from the single image I have that could be loosely described as a 'portrait', I'd say the Sigma will also be portrait lens, judging from the bokeh.
(Note: I mostly use my 105VR as my preferred portrait lens. Not that it's a bad macro(1:1) lens, it's probably more that I was expecting more from it. It is a sharp lens tho, and for sure you'll be impressed by it, but I believe that after a while, you may find yourself in a situation where you may want more from it.