As stated by other users, without a BIOS that has an option for you, there's no way to allocate more VRAM for the APU.
Laptop manufacturers rarely give flexible BIOS. They usually go with a "Tatooed" BIOS so people can't explode a laptop and call for warranty afterwards. They usually gives some options, like VRAM settings, for gaming oriented devices.
Yes, unfortunately, the Vega 8 could support much more VRAM, but it depends of the manufacturer support.
I'm not sure if someone from the community would be able to make a custom BIOS, and unlock "without risk" the vram settings.
You have to be conscious that *unlock* implies the settings are already included / coded in the BIOS. If it's not the case, it a whole next level of sh!t.
But hey, as long as there's life there's hope.