As a follow up, I wanted to post some of the documentation from the Vray experts at Chaos Group. From the official documentation:
VRayFastSSS is a material that calculates a sub-surface scattering effect. Since it uses a simplified algorithm, it is typically faster than the sub-surface scattering option of VRayMtl and may be easier to control. Note thatVRayFastSSS does not include diffuse and glossy effects. To add these, create a VRayBlendMtl material withVRayFastSSS as a base material and a VRayMtl as a coat material.
VRayFastSSS simulates the effect of three sub-surface layers - a shallow layer over a deep layer over a back-scattering layer. You can control these separately to achieve different looks for the surface.
Prepass rate - VRayFastSSS approximates the sub-surface scattering by precomputing the surface lighting at different points on the surface and then interpolating between these points. This parameter determines the resolution at which surface lighting is precomputed during the prepass phase. A value of 0 means that the prepass will be at the final image resolution; a value of -1 means half the image resolution, and so on. For high quality renders it is recommended to set this to 0.
Interpolation samples - this parameter determines how many samples will be used when smoothing the surface lighting. For very translucent surfaces, there must be enough samples to blur the lighting across a large portion of the surface and so you may need to increase this parameter. Less translucent surfaces can do with fewer samples, which also makes the calculations faster.
Diffuse roughness - this parameter controls the surface roughness for the material. A value of 0.0 produces a diffuse material, while higher values give the surface a translucent quality.
Shallow radius - this determines the radius (in scene units) of light scattering for the first (shallow) sub-surface layer. Smaller values produce less blurring of the surface lighting and are faster to calculate. Larger values blur the lighting. Note that increasing this parameter may also require you to increase the Interpolation samples.
Shallow color - the color for the shallow sub-surface layer. Note that this color is also used as a filter on top of the Deep color and the Back color (e.g. if the Shallow color is pure white, no deep- or back-scattering is calculated). The color can also be controlled with a texture map.
Deep radius - determines the radius (in scene units) of light scattering for the second (deep) sub-surface layer. Typically, this value would be several times larger than the Shallow radius. Note that increasing this parameter may also require you to increase the Interpolation samples.
Deep color - the color of the deep layer. Note that this color is also used as a filter on top of the Back color(e.g. if the Deep color is pure white, no back-scattering will be visible). In a similar manner, the Shallow coloris used as a filter for the Deep one. The color can also be controlled with a texture map.
Backscatter depth - a maximum depth (in scene units) at which the backscatter effect will be visible. A value of0.0 produces no back-scattering.
Back radius - the radius (in scene units) for the back-scattering effect. Larger values blur the back-scattered light, but may be slower to calculate. Increasing this parameter may also require you to increase theInterpolation samples.
Back color - the color for the back-scattering effect. Note that both the Shallow color and Deep color are used as filters on top of the Back color.
VRayFastSSS uses the V-Ray prepass system to simulate and interpolate the sub-surface scattering. During other GI calculations (e.g. light cache or photon mapping), the material is calculated as a diffuse one.
VRayFastSSS does not require that the object has any actual volume; it relies purely on surface lighting.
Interesting Notes From This:
The settings are all pretty self-explanatory, and you can see how they work in the video. The one thing I do find quite interesting is that these SSS materials from Vray have no reflection or glosiness to them. I have found that in practice, if you want reflection, it is necessary to do exactly what they suggest and use a blend material. I am not sure if I mentioned that in the video. I also think it is interesting that the shader relies entirely on surface lighting, therefore, your objects require no volume. I will have to keep this in mind when I am trying to create depth in water or something. This shader works great for that, by the way, but apparently it can just be a flat plane. Awesome.