If you're using Vray then you don't have to concern yourself with the metal/rough workflow. If creating your textures in a metal/rough environment like substance then you can use the Vray export preset which will convert your maps for use in Vray: diffuse/reflection/glossiness/etc. the roughness channel in Vray is to map…
Firstly I don't feel this is off topic at all because it is directly about non-black non-white metallic values, but you seem to still be confused. I really don't want to drag this out, because it goes over artists' heads and is not very helpful information when they can't do anything other than "Well switch to Specular".…