GPU Video Memory

I have an iMac Pro with the Radeon Pro Vega 56 8GB graphics system. I am considering getting the new MS Flight Simulator which needs 4GB of Video RAM for "reasonable" performance. When I check the Parallels configuration, there doesn't seem to be any way to set the video memory higher than 2GB - and there is a note that says Parallels will use system RAM for graphics, which will won't work here, I think. A couple of questions: Can Parallels access the video RAM on my Radeon Pro Vega graphics? How can I adjust how much of the video RAM that the VM will use?

David