Ok, i got their reply. It seems VRay 2.4 just produced some random result, which in some cases looked more correct, although really it was not.
Instead VRay 3.05.04 now should render the same result in RT... except if Forest Color tint or random texture options are used, because this plugin is not fully supported in RT: it just renders the default texture.
Our grass presets use Forest Color, so always will be some difference between VRay standard and RT. As reference, i've attached render samples of the "Common grass" preset.
I hope all is clear now... there was a bit of mess with issue.