Will the gtx 770 2gb of vram be enough for single monitor 1080p at high- very high settings for demanding games for the next 3-4 years?
Or will the gpu's processing power limit be before the vram does?
To answer your question yes, but I would recommend an r9-280x 3gb. It is usually cheaper, and competes with the 770 in most games. But if you really want that 770 instead, its still an amazing card and yes 2gb is enough
for now 2gb is enough, for the next 3-4 years i cant answer that. It would only be at the highest detail settings at 1080p with AA on that you would crack the 2gb barrier in future games. at the very least you should still be able to play games on medium-high settings in 3 years time. I would say as you suggested, gpu processing power will become a limit with either card before vram does, at least at 1080p resolution.