I think you don't to have at least 512 mb of vram for all the advanced gpu features like perspective warp to work.
Actually 512 mb of vram has always been the minimium requirement for photoshop cc, although some of the gpu features still work with only 256.
Follow the links below for more info:
Photoshop Help | Warp perspective | Photoshop CC
Let me know if you have more questions on this.
Cheers~
-Mandhir