V-Ray volumetric lighting speed up render with black shaders

[newimaging]

I was just told by a friend that he remembered when using v-ray years ago that there is a button that lets you turn all objects in the scene to matte black for rendering a separate pass with volumetric light. Therefor speeding up the render a lot. He then composited it via a screen effect in AE.
Was wondering if anyone can guide me towards this. Other option is to turn every shader black in a backup scene.
Pretty new to this, but the render times are killing us even in a preview image otherwise.

Thanks for any help.

[sharktacos]

Don't know about that, but you could just make a render layer for it.

[squintnic]

override with black ?

[sharktacos]

yup. Material override.