Do you have an integrated GPU or dedicated GPU? You can check this with GPU-Z.
Usually, GPUs "borrow" system RAM to operate, even dedicated chips cache some of the RAM in case they need it. If you install more RAM, then they will be able to cache more of it, but there is a limit to this. You cannot actually add VRAM, because that would imply soldering VRAM chips onto the motherboard (if the MB allows it).