UPDATED
Hi @Doupago,
About direct access to Mac vRAM, no, Mac applications do not have direct access to computer's vRAM. OpenGL and Metal manages allocations and virtualizes video memory for applications' use. Parallels Desktop uses OpenGL API to implement 3D acceleration.
About memory allocation. Yes Parallels Desktop allocates graphics memory from Mac system memory, except for a case when video memory is set to 'Auto'. In this case Windows itself allocates memory for graphics purposes from guest system RAM (which is allocated from Mac RAM), but not more than a half of assigned RAM.
So if you Mac video card has 3GB of vRAM, but your virtual machine has 4GB of System Memory (RAM) assigned, Windows will use only up to 2GB of video card's vRAM. To use all available on Mac vRAM, the amount of assigned RAM should be twice higher - 6GB in this case.
Last edited: Nov 20, 2018