Thanks for the question. We have encountered any issues with V-Ray 5 and Forest colour. If you have a sample scene we could check it might help us to identify the issue.
Regarding your question about VrayMultSubTex , this feature is available in the Forest Pack 7 beta which is available not for users on an active maintenance plan.
"- Added support for 'Instance ID' when using VRayUVWRandomizer and VRayMultiSubTex with Forest Pack"
This won't give you quite the same features as Forest Colour of course, which allows you to randomise and tint by element as well as other advanced features.
You can see a full list of what's new here: https://forum.itoosoft.com/announcements/forest-pack-pro-7-0-0-beta-7169/
Hope that helps,