What is the addon that does this?

It takes the thread title and part of the post, and places it above the thread. Here is an example from another thread here:

"This is a discussion on Mark Forums Read Not Working... within the General Discussion forums, part of the vBSEO SEO Plugin category; I upgraded to the latest vbseo tonight and now my mark forum read feature does not work... When you click ..."

Is this something in vbseo, or a different addon?

Those are the vbseo Relevant Replacements.

Do I have to manually edit the templates to insert the <!--VBSEO_RR_1-->?

I added the example code at the bottom of the Relevant Replacements and it didnt seem to do the trick.

Yes, you need to put that where you want it to show up.

Generally, most people put it in the ad_navbar_end template so you don't have to revert showthread/forumdisplay on upgrades.

I got this sorted out yesterday, and it works like a charm once you get it right. Check my Critique thread, Marco was a dream in explaining it quite clearly.