The tammy, (its VC not VR, as VR is a Nikon term for stabilistion, like IS is for Canon ) is pretty ok for the price in the 70-200mm range, but quality drops when racking to 300mm.
I'd agree with Rick, if the Tammy is around your price point. Spend a wee bit more and go for the Nik
I saw one comparison by someone who has both where the Tamron was significantly sharper at 300mm. FWIW.
I don't remember but it was on DPReview. Although some people suggest the Nikon after comparing both... I just haven't seen any comparisons showing the Nikon is better. However some Tamron 70-300 examples show variable exposure depending aperture (i.e. over or under exposure depending on aperture).
I think this is one of those exceptional third party lenses where it's a difficult decision since the QI is so close. One of the problems with all reviews is they tend to be bias and attempt to prove a preconceived result. Another problem with advice and reviews is , for the most part, we like what we own and want you to experience our satisfaction.
I've found the most valuable reviews come from those that have owned both lenses in question. The most popular lens is not always the best, but it gets the most hype. I went through this when deciding between the nikon 18-200 and 16-85. The 18-200 is very popular and held in higher regard than the 16-85, yet most reviews conceded that the 16-85 was sharper. After reading reviews from owners of both, I went with the 16-85.
I have the Nikon, so from my positive experience it is the one to get, but the Tamron may certainly be just as good. The only negative I read about with third party lenses is resale value and getting "a good copy". I would suggest if you go with the Tamron, you buy it in a store where you can test it first and walk out confident that you have no quality issues to worry about.
I have the Tammy 70-300mm VC lens, and love it. When I was looking, I asked about it at Nikon Cafe forums (hadn't found this place yet). Most of the folks there told me the Tamron was sharper than the Nikon 70-300 VR lens, in the 200-300mm range. Since I was getting it for wildlife shots, I figured it would be set to 300mm most of the time, and it is. It is good and sharp. I've not used the Nikon 70-300mm, so I have no personal experience in comparing the two. I just took the word of several people that had used both, and preferred the Tamron. I'm very happy with it. I got mine at Amazon.com. Right now, it has a $100 mail-in rebate, which gets the price down to $349. When used on a D7000, the f/4-5.6 does very well in low light too. It also has a better warranty than the Nikon lens. The VC (vibration compensation) works very well. I like it better than Nikon's VR, but the VR is a little quieter.
Good luck!