As far as I'm aware, most people view Bulletin boards in 'flat view' which basically puts everyone's post in timestamp order, regardless of who they're replying to. (I never met anyone who used 'threaded' even where it was available.)

Threaded view means that instead of using 'quick reply' the user has to specify to whom they are replying. Then the replies are 'threaded' under the post they are replying to.

There are two problems with enabling this: one is that I'm not sure whether vBulletin even has that option any more. The other is that 99.9% of posters will reply using the 'quick reply' box, so the answers don't get threaded anyway. (And if you want to reply to someone specifically, you use the quote function instead, as I just did. )