Question UHD 770 Video Memory for Dell XPS 8960

You can define the VRAM amount from BIOS. Anywhere from 128MB up to 2GB is common. Or set it as Adaptive (if there is such option), so that CPU allocates/removes VRAM amount based on the needs.
 
In your "Run box " on your start menu type DxDiag.exe you can see how system is spreading out the memory under the Display tab.

What I'm not sure is how using the CPU's built in GPU plays out when reading video memory but it should still read how it breaks down with DxDiag.exe