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With that amount of Vram left why doesnt the game use it for more power to get better frames?.
Think of VRAM more as something that unlocks the ability to use larger textures and resolutions. As long as your card has it covered, there's no problem. Even if you do run out of VRAM, it's not necessarily going to wreck your game. First of all, many games allocate more things in VRAM than they actually use, so your VRAM shows full but really only part of that is being deployed. Secondly, as mentioned here, there are opportunities to swap things from main (slower) system RAM. Although this does create a large bottleneck it's not necessarily instantly catastrophic - that depends on the game and the amount of data.
With a 8gb 1080 you have enough VRAM for most situations, even at 4K. There will be some games and settings that cause hitching now and then. Don't worry if the VRAM isn't full - in fact that's probably a good sign. Even if it is full, try the game first before lowering settings. Different engines allocate in different ways.