Its not a thing. Can't be done. The operating system will take whatever memory the graphics needs and it happens so fast its hard to track. Integrated Intel HD video can only use so much memory and your video is performing as well as it is capable of doing with the current setup.
I could get technical but keep in mind it does not hold onto memory for more than a small fraction of a second. The video system is just not powerful enough to use much memory. It can't pump pixels through fast enough to need much more than 128 megs of vram. That is why it is no good for gaming. Powerful video cards are pumping lots of pixels through very fast and it needs somewhere to store them before they are displayed on the screen and that is what vram is doing.
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