Using Falloff map to simulate Roughness in VrayMtl?

[Alex_M]

It looks like Vray GPU unfortunately doesn't support the Roughness effect in VrayMtl which I use more and more these days. So I was wondering if I could actually approximate and essentially fake the Roughness effect by using a Falloff map with a darkened version of the diffuse texture/color for the center and lighter version for the perimeter. Or is there more to the Roughness effect that can't really be replicated with a Falloff map? What do you think? I'd love to hear how you'd approach this or if you have any other suggestions.