Unless there is a way to retroactively subscribe to all threads I've posted in, that workaround isn't going to help (not to mention it may not sort properly either). It should be an easy fix to sort search results correctly if the folks behind the scenes know how to fix what is likely a bug in this inistall of vBulletin.
Edit: this thread isn't bubbling up either. Here's is a screen print demonstrating the issue.
http://imageshack.us/m/39/6493/notsortingbylastpost.gif
As you can see it is not sorting by last post (and this thread is sorting too low to be captured in the screen print).