How to check how much "free" iGPU VRAM the system have in MB?
You can check this using the iStats Menu program, which is a paid app that you can try for free for 14 days:
https://bjango.com/mac/istatmenus/
Note that you'll probably find that the VRAM is maxed out, and there's much you can really use this metric for unless you happen to be a developer actively developing your own software using the GPU. In that case, you should use the free Instruments tool from Apple instead.