I have noticed on several successful vbulletin forums a sort of feature where when a thread is posted, on the very bottom there is a list of five or so threads with similar keywords etc. So if the thread is about SAT best math book, there would be a list of like 5 threads on the very bottom on the same topic.
I think this would be especially beneficial to a forum like this, where there are many duplicate threads on similar topics that can prove to be invaluable to members searching for info.
I’m sure the hack/customization wouldn’t be hard to find, many vbulletin forums have it.