the GLX_MESA_query_renderer OpenGL extension was introduced in 2013. The patches are out. Now it’s your turn.
As VRAM is only being accessed through a MMU, one can only get the size of the memory window being used without directly interacting with the driver.
What this means is technically every linux user is stuck with 256ish mb of vram without regard for what you actually have.
Since for some god forsaken reason I am not trusted to enter my own vram amount, I fully expect a time frame to deliver this important upgrade.