I have GTX 1060 6GB and when I play GTA IV, I can only use 3072 MB VRAM. I don’t use dual boot, migrated completely to Ubuntu today. I didn’t have this problem on Windows. The driver is “NVIDIA driver metapackage from nvidia-driver-470 (proprietary, tested)”.
I suspect this is due to dxvk having a tuneable for crash-protection. https://git.uplinklabs.net/mirrors/dxvk/commit/b25d6ba615a8522f1d056413e837228cbcf682fa