It is not recommended to use VRayMultiSubTex with Forest Pack due to the way the plugin redistributes IDs. If you must use it there are two options.
Firstly, you can disable Forest Pack's automaterial features by unchecking Consolidate materials. Be aware though that you will now need to manually create a single multi-sub object material, assign IDs manually and assign it directly to the Forest Pack object.
Alternativly, if the number total of items is not huge, use Forest Tools
to create instances of the source objects. Since the IDs are not modified, you can apply VRayMultiSubTex without changes.