brucelay
How would you do it in RSL surface shader with Vex Builder?
The EdgeFalloff VOP is a subnet – if you go inside it (select and press Enter), you'll see it's really just a network of VOPs, all of which are available in the RMan context. So you can either copy/paste the VOPs over to your RMan shader, or open up its “Operator Type Properties” dialog, select the “Basic” tab, change the “Save To Library” field to something local, append the string “rsl_*” (without quotes) to the end of the “VopNet Mask” field, and hit “Accept”. Now this VOP should be available in the RMan context.
Last but not least, EdgeFalloff is similar but *not* the same as Fresnel, and Fresnel *is* available in the RMan context.
P.S: Since you seem to want to learn PRMan, I thought you might be interested in seeing what the EdgeFalloff VOP *really* looks like in Renderman Shading Language (RSL):
color Cfall = basecolor * pow ( abs ( normalize(N).normalize(I) ), falloff );
If we ignore RIB for a moment, then we could safely say that RSL *is* Renderman. Worth learning even if you choose to use VOPs.