When using either Raytraced (solid) or Raytraced (refractive) mode for the Single scatter parameter, you need to use VRayShadowsfor standard lights in order to get the correct results.
The VRayFastSSS2 material computes sub-surface scattering only during the final image rendering. During other GI calculations phases (e.g. light cache), the material is calculated as a diffuse one.
For the reason explained above, VRayFastSSS2 will render as a diffuse one with the progressive path tracing mode of the light cache.
You can layer several VRayFastSSS2 materials using a VRayBlendMtl material in order to recreate more complex sub-surface scattering effects. In this case, any raytraced single scattering are only calculated for the base material, but multiple scattering, reflections etc. work correctly for any layer.
