I don't see this as a an issue. I currently have an RX 480 with 8 gigs in it because I run several games which already hit the VRAM limit, most notably modded Skyrim.
The FPS difference is null and void since vast majority of the time I end up capping it at 60 anyways otherwise bad crap happens up to and including crashes and games unplayable (as to what kind of crap and details, feel free to google it if curious, answers not hard to find).
So given that anything above 60 FPS is kinda useless and the cost difference between 8 gig video cards from AMD and Nvidia its a no brainer. If need to, maybe 1.5 or 2 years from now I'll buy another AMD card with whatever the up to date GPU with most VRAM will be at that time.
Also the rest of the system does make a difference. Funny a poster above mentioned Fallout 4, not only do I not have any issues with running it at max settings @2k, but just like Skyrim, its modded, so extra VRAM is where its at. 10 or 20 FPS this or that way makes no difference if the game or its mods are completely unplayable due to lack of VRAM, and I sure aint gonna spent the $$ Nvidia wants for 8 gigs.