VRAM has become a marketing issue.
My understanding is that vram is more of a performance issue than a functional issue.
A game needs to have most of the data in vram that it uses most of the time.
Somewhat like real ram.
If a game needs something not in vram, it needs to get it across the pcie boundary
hopefully from real ram and hopefully not from a hard drive.
It is not informative to know to what level the available vram is filled.
Possibly much of what is there is not needed.
What is not known is the rate of vram exchange.
Vram is managed by the Graphics card driver, so there may be differences in effectiveness between amd and nvidia cards.
Here is an older performance test comparing 2gb with 4gb vram.
http
/www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/
Spoiler... not a significant difference.
The GTX970 would be the stronger card in any environment.