Hi, we worked with Chaos Group to resolve a few issues in previous versions of Forest Pack which involved changes to V-Ray as well as our plugin.
As a result currently supported V-Ray versions are 2.x, 3.1, 3.2 and 3.3.
As far as I know, if you're already using any of the V-Ray 3.x version upgrading to the most recent version should be free of charge.