Macs such as yours adjust the video RAM according to the amount of RAM installed, but will never apply less than 256 MB in the stock setup (2 GB). If this amount is to change, you need to upgrade the RAM (which may or may not increase the lower limit) and make the system work hard enough to call for more RAM. This is, in theory, how dynamic video systems are supposed to work. On some Windows-based notebooks, there really is an option in CMOS Setup that can lock the initial amount of VRAM reserved by the integrated graphics processor (IGP), which may also raise the max memory ceiling for the IGP. It's up to the vendor to determine whether it wants to let users adjust VRAM this way.
Apple provides no mechanism to allow end-users to modify EFI ROM settings beyond what lies in System Preferences. So there's not really a way to tell the machine how much VRAM you want.
But do take comfort in this: your model is confirmed here to accept as much as 8 GB main RAM, which is in many ways better for game performance than VRAM.
Nate