There's something that you're always missing about VRAM. First of all, no game uses 8gb nowadays, not even Flight Simulator at native 4K:
https://www.youtube.com/watch?v=lRffDgqngyk
Also, there's less real raw power on VRAM on AMD options because they use GDDR6 instead of GDDR6X, and when comparing any benchmark at 4K AMD has a huge bottleneck there that is gonna be solved with smart access memory, a feature that's also coming to nvidia and to directX itself.
So, no, AMD is not more powerful.