vBulletin's Social Groups are documented here.
The social groups permissions appear to only affect writing permissions, not reading permissions.
It feels like that new feature of vBulletin isn't quite cooked yet.
I can create private forums, but those can't use Social Groups to define permissions. Forum permissions use vBulletin Usergroups, not vBulletin Social Groups. vBulletin Usergroups are documented here.
Hmmm... We could create a "Blackhat" subforum, but I don't think it would be possible for me to delegate membership management. I think I would have to add each member by hand.
Heck, there's probably a vBulletin mod that solves all these issues.
We're going out for the evening. I'll take a deeper look tomorrow morning.