Migrate a vBulletin 4 forum to Discourse

Sorry. I don’t remember well enough to explain. This is about all of the free help I have to offer on this one.

Of course. See How to use category security settings to create private categories - faq - Discourse Meta

Some importers endeavor to import groups, but few of them know how to apply those permissions to the categories that get imported. You’ll need to fix those up by hand.