Bad news I'm afraid. The anticipated mark threads as solved plugin has been installed, but in the attempt to get it enabled, most of the solved prefixes from already marked threads have disappeared. Several factors were involved, but the bottom line is that this included human error, and for this we apologise.
The frustrating news is that the plugin is still not working. Long story short, but we need access to code which only vbulletin licence holders have access to. Obviously, Canonical have a licence for the use of vbulletin for ubuntuforums, but red tape is getting in the way of our seeing the code we need. This is largely out of our control but we are working on it.
In the meantime, threads can still be marked as solved as described here.