Increase VRAM on Mac mini Late 2012
You can indeed increase the amount of VRAM in certain Apple models.  In fact, you have already done that for 1 or 2 times.


2012 Mac Mini 4GB allocated 768MB to VRAM in it's original release state.
Upgrading the RAM back in 2012 to 8GB would have given you 1024MB.
Keeping the RAM at 4GB but upgrading to OSX 10.9 would also resulted in 1024MB.
Following your claim that you have 1536MB available this might be the result of either OSX 10.9.3 or because of the 2nd 8GB.  I don't know.
I probably still have a 2012 Mac Mini somewhere in the cupboard and will test this soon.
2013/2014 MBA and MBPr models came with 1024MB out of the box.
This was also increased to 1536MB after installing OSX 10.9.3


There might be more models that have received a free VRAM upgrade in the past but I have no direct records of these.