Cycles multiple material and multiple importance sample light I'm having a problem with multiple materials and multiple importance sample options on the light.

I make a vertex color to assign the different material to the vertex. Node setup is in the screenshot. Also, I choose the multiple importance sample option on the sun lamp.

The problem is the shadow from invisible part of the prism.
I need to make a shadow only from a visible part. Also, there is no way to call a vertex group by name, only by color, right?
The problem was that there were zero diffuse reflection bounces. This value can be set in the Light Paths settings.