HA! Doing the happy dance I found the links I was looking for. Check this out, they have lots of good info here for making your vBulletin SEO ready without other mods... http://www.vbulletin-faq.com/forum/f...our-vBulletin&
The thing is if you have had this site up for awhile and have many threads, articles etc which it looks like you do, then now going back and adding SEO can be a bit more complicated than if you did it to begin with when you first launched your web site. So you would not only have to put the SEO stuff in place but also re-route all your current traffic to the new url's if you plan on installing something that changes the URL'S. This is something to think about as it can be a bit more work in the long run.
I'd suggest looking over the vb-faq link that hm666 posted.
There are several things you can do that aren't "changing the url", like working on your header tags for starters.