I was primarily referring to a forum setting, RSS is more of a headline provider (at least in my opinion). If it was a blog, I could see delivering all the content the site has to offer in a full feed, but in a forum the purpose is to get people on the site participating. I have built and ran several websites (forums) over the years, the largest having 112,000 members and about 10.5 million posts, and have found this to be true almost across the board.

To answer your original question though: I don't believe vBulletin allows for all content to be delivered via RSS out of the box (at least not the FORUM...possibly the blog, probably for the reasons I've already mentioned). I'm sure it's not hard to modify, but that's not something I'm really interested in doing.