When using V-Ray renderer, the Raytrace
mode will be used for Fake Shadows
(XShadow works exclusively with the Scanline renderer) and the light which casts raytraced shadows needs to be selected (VRaySun).
Let me mention, that raytraced shadows require Forest Material
to work, because this plugin does the job of hiding or rendering the shadow / texture for each plane. Be sure that the material applied to the Forest object uses the Forest Material
as a base.
Attaching a sample scene showing this set-up. Hope you'll find it useful.