They rely on normal system RAM. Which is exactly why you would want to use faster system RAM than usual, because DDR3 has so much less bandwidth than GDDR5.
By the way, the VRAM is DRAM too. Today VRAM just indicates it's RAM reserved for graphics/video, DRAM indicates the (broad) type of RAM. DDR3, GDDR5, system RAM and VRAM are all DRAM in practice.