What you do here is create groups, then add the selected users to the groups. They will have to log in to view.
The other aspect is the setting of each forum, you can set them to login only to view, or not, in which case you don't need to create groups. You create forums and set the per-forum permissions.
The only time I've edited vBulletin software is to get the pages to validate under html 4.01 instead of XHTML, and not a lick of it is XHTML output, it's all HTML.
Does that answer it? :-) The Vbulletin forum is very helpful, however, tread carefully, there are many flamers over there.
An example of both is when posed a few questions about this there, I got jumped like no tomorrow - "What is this HTML you speak of?"