Yes, "hide planes..." must be disabled to cast the shadow in VRay (this is fixed in 3.2b).
You cannot open the scene because is for Max 2009, i've upload it a version for Max 9 with this post.
The problem with illumination is that each render engine operates in a different way. Self-illum works with Scanline, but is inverted in Mental Ray / VRay (100% looks as black). We must test the plugin with each possible combination and adapt it. Until now, we have introduced many changes for Mental Ray in 3.1, and now we are working with VRay for Forest 3.2.
Some tricks for Forest 3.1 + VRay (used in this sample scene):
- Instead using Self-illum, use Diffuse Map->Output->RGB Level (i use 3.0)
- To avoid the Self-shadowing issue, de-check "Receive Shadows" in the Forest object (right menu->Properties)
All these problems must be fixed in 3.2, but this is a endless work since each version of Max/Mental Ray/VRay/(put here you favorite render engine)... introduces new changes. My wish is that users post here their problems with Forest, because is the only way to suggest a workaround and fix it for the next release.