Sometimes it is a matter of luck and timing. We were there in October, when we first got to the south rim there were scattered showers moving through the area which made for some interesting conditions.
The Tamron is an FX lens. I use it on a D5100. The VR on the Tamron is very good. I find it quite sharp. I have not used the equivalent Nikon lens for comparison, but I am extremely happy with this lens.
I had the 55 - 200 lens, I wasn't happy with it so I sold it and bought the Tamron 70-300.
My bucket list includes the Grand Canyon and the Southern Utah National Parks.