2. Some other renderers use 3 values for the scatter radius instead of a single float. The idea here I think is to have the red wavelength pass more light though that the blue or green.You do have three values for the scatter radius - the meanFreePath is a color, so it has three separate values. Although it is represented as a color, the meanFreePath is actually the scatter radius for the red, green and blue light components.
Yes, will make a note about it.
1. Am I correct that the sss part of the VraySkinMtl is made of 3 layers of sss2 added together with differing values for the scatter radius (in OSL the meanFreePath)?No, not exactly. The vray_subsurface() closure corresponds to the VRayFastSSS2 material. The VRaySkinMtl material is somewhat different; there I tried to follow the way it is implemented in other renderers and it works pretty much in the same way. It is calculated internally as three grayscale vray_subsurface() closures multplied by the color of the respective component (with the VRayFastSSS2 material being a special case where the three components have red, green and blue colors). The VRaySkinMtl is also optimized to compute the three sub-surface components together, whereas if you just add them in an OSL material, you could get the same result, but it would lead to way more calculations.
Best regards,
Vlado