Render a test with White Balance set to Custom with pure white color.
Let the rendering go on at least until the pixel values stabilize (the Denoiser works at least once..).
set the white balance custom color to a color sampled from the VFB at a point that you know should be appearing completely neutral (neither warm or cold)
* Each time you want to test different sample locations you need to sample the pixels from the test rendered with custom pure white.
The procedure in Maya is identicle, Add V-Ray Physical Camera attributes to the camera and enable them,
Create a test render with pure white White-Balance Color,
Sample the white/grey surface color in the V-Ray VFB (Render Window) to the White-Balance color, and render again.