If the option or BIOS Setting is missing to increase or decrease the IGPU vRAM that means the manufacturer has set the vRAM to be permanent and can't be changed.
Generally if there is a BIOS Setting you can go from 526MB to 2GB or larger depending on the Manufacturer.
It really doesn't matter because Windows will automatically adjust the amount of vRAM the IGPU needs to run a specific App or Windows or video or game.
Besides if you were able to add more vRAM Memory that vRAM Memory is taken from your System RAM Memory amount. So if you have 8GB of System RAM Memory then with the IGPU vRAM Memory you now have 7GB of System RAM Memory to use.