for vBulletin, when you purchase it, has a complete individual forum permissions... each sub-forum has it on its own... moderation, subscription etc... no matter how many depth your forums has, you can handle each of them individually... if you do not touch them, they are handled by their parent...
you can see the tutorials here:
https://www.vbulletin.com/docs/html/...anaging_forum_
and good luck, we'll be happy to help you with your future licensed vBulletin !