vRAM is Video Ram for use by the Graphics card. It is not main memory or system RAM.
If you need more system ram, you have to purchase it. If you need graphics vram 2-4 GB is sufficient. But "future-proof and get the 4 GB card.
Look at it this way. The GPU (video card) is used by Photoshop to do some stuff. The GPU works only with vRAM. While the rest of the computer works only with RAM.