Hi, Personally, I would recommend to use the FSP GOP, which should/could work with even the stock VBT in "*FspBinPkg/SampleCode/Vbt/". You can also customise it for your board using Intel BMP (Binary Modification Program) available online.
However, if you want to use the VGABIOS, you shouldn't need to set more than `VGA_BIOS_ID` in the mainboard config (as far as I know) and perhaps `VGA_BIOS_FILE`. Leave the VGABIOS in the appropriate folder and `VGA_ROM_RUN` can be set by menuconfig.
I can think of two issues: 1. The VGABIOSes all have the DID "0x0406," but this is not the DID of the iGFX. 2. The VGABIOS may issue interrupts. You can see if it does by opening it in Intel BMP if you have the corresponding BSF script file. If it does, you should `select INTEL_INT15` and have the mainboard install the handler (for examples, see the Thinkpad's' "mainboard.c"). However, coreboot's handler does not handle all interrupts. I haven't tried this myself.
Best regards, Benjamin