I think it's worth pointing out some of those technical differences.
In vBulletin 1, there were separate database tables for forums and categories, and they were genuinely different items. With vBulletin 2 and our introduction of infinitely (theoretically) nestable forums, it made sense to merge forums and categories into a single data type for performance reasons. Therefore, in vBulletin, a category is a forum, it's just a forum with 'Can contain posts' turned off. That's the only difference between the data.
Conversely, XenForo places forums and categories into a flexible node tree, into which any kind of data can be placed. In XenForo, categories, forums, links and pages are all nodes, but categories and forums both extend the node concept in different directions. They have different options, permissions and all sorts of other differences that means that unlike vBulletin, it's far from trivial to turn a forum into a category, and it's certainly not something you'd want to do on regular basis.