Hi Eugen-
You can create your own Scattering Medium pass with the following procedure:
1. Set the Scattering Medium Transmission and Albedo to black.
2. Set the Environment Background Color to white, and turn off Ground Shadows.
3. Render image and then invert.
The example images below are quick renders at 64 samples each. They are:
1. Bunny VDB beauty render.
2. Bunny VDB mask render.
3. Bunny VDB mask render inverted in Photoshop.
4. Result of mask used as Layer Mask with a Hue/Saturation Adjustment Layer applied to Bunny render.
5. Smoke VDB beauty render.
6. Smoke VDB mask render.
7. Smoke VDB mask render inverted in Photoshop.
8. Result of mask used as Layer Mask with a Hue/Saturation Adjustment Layer applied to Smoke render.
9. Result of mask used as Layer Mask with a Gradient Map Adjustment Layer and a Hue/Saturation Adjustment Layer applied to Smoke render (also used to mask out and change the background).
Hope this helps

Cheers,
Eric