Are you using the latest version V Ray 3.6? ifso your answer is can be found here:
https://www.youtube.com/watch?v=HZ_FeuG9WSE
you need to go to your settings tab and pick the GPU button then click on the 3 dots and look to see if your graphic card is showing up there , you can then pick its on or off. when on a check will show up next to name.
VR-GPU.jpg