70-300 Tamron sharpness

None of the consumer zooms are super sharp at full telephoto. The Tamron is one of the best out there, but there can still be variations in copies. Exchange yours for another and if it is not sharp either, try another brand. My 70-300 VC is nice and sharp, but goes just a tiny bit softer at 300mm. Less so than the Nikon version I tried.

One thing you need to be aware of is that your shutter speed should probably be a bit faster...you are shooting at 450mm equivalent, and even with VC on, you should have been closer to 1/320 or better yet, 1/400 or 1/500. Tamron's stabilization is not as smooth working as Nikon or Sigma, so the "jerkiness" of the VC can cause issues sometimes. That is where the faster shutter will be to your advantage.

I also find that f9 to f11 is noticeably sharper than f8 . You will need to bump ISO a bit to get these settings, but your results should be better. Also make sure there is no slight front or back focus going on...my copy was only good on my D700 until I fixed my D7000 AF. Now it is stellar at any focal length on either body. And it nails focus every time.
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K.B.