Thanks for your question and the scene file.
There are two issues here that are preventing this from rendering.
Firstly, when nesting Forest Pack objects with V-Ray it is currently necessary to change the mode from Automatic to Mesh. This will force the nested Forest Pack object to a single mesh that can then itself be scattered. In previous versions of Fp, this happened automatically, but a recent update has necessitated this setting to be changed manually. We should fix it soon.
Secondly, because the nested Forest Pack object is collapsed to a single mesh, then you need to be careful with the polygon count. At present you are scattering 3264 objects each of which is 29,410 polygons. That means that the patches poly count is going to be 95,994,240. That's very high, you should optimise this by reducing the size of the patch and scattering more of them in the second FP object. Also, with high polygon nested patches, you may need to increase the Display> Max Faces Limit, or remove the restriction completely by setting it to zero.
I hope that helps,