In my experience the spotting scope and the 15’s serve a Different purpose. I have glassed using the grid method with my 10 x 42‘s and my 15 x 56’s. I turn up a lot more deer with my 15’s at greater distances obviously. If I want to get a real close look at that animal then I would pull out the spotter.
It also depends on the terrain your hunting. I am spotting deer out at 1.5 miles with my 15s. You can see the bulk of the animal, but cant really See the important details. The Binos would be much more comfortable than the spotting scope for long hours glassing.
Yes on the adaptor, but the vortex tripod adaptor came in the box with the Vultures. I did not have to buy that separate.