Post Rating
 

Basically, similar to Luvdisc/5-Star/upvote/downvote threads, I think it'd be a lot more useful to have a system available for individual posts, the main reason being to recognize and encourage good posters to keep posting, esp. in threads where most of the content is crap but there are a few diamonds in the rough. I understand there'd be coding issues in implementing it, but I'm wondering what the general idea is.

vBulletin does not come with this sort of functionality. There is a "reputation" system, but that is not quite what you're asking for, and is in general a dreadful thing to deal with.