Is there anyway to ''borrow'' from my 1.5GB of VRAM to be used for my RAM?
Otherwise, am I flat-out stuck with my 4GB of RAM?
Too bad it's soldered down. :(
Unfortunately, your question is based on a misunderstanding of how your particular MacBook Pro's VRAM works.
Based on your question details, I assume you have a Late-2013 13" Retina MBP. These models came with an on-board Intel Iris 5100 graphics card. This will actually use up to 1.5GB of your computer's 4GB RAM for its graphics processing.
In other words, the on-board Intel Iris 5100 graphics card does not actually have 1.5GB of VRAM that you could try to borrow as RAM.
Since your RAM is soldered on, if you were to keep this computer then your best bet would be to try and reduce the amount of VRAM being borrowed from your computer's RAM. Whether this is even possible will depend on how you use your computer (e.g. if you have a 4K external display connected then it'll be using more of your RAM as VRAM, if you have no external displays and do nothing but check email all day then you're probably already using as little RAM as possible for your VRAM).
There is also the option of using an external graphics card connected via your Thunderbolt port, but to be honest this may not free up much of your RAM if not much of it was being used as VRAM to begin with. And with only 4GB RAM, I'd be looking at the option implied by Steve Chamber's comment (i.e. sell your MBP and buy another one with more RAM).