Try going to
DynamicDrive.com as Shiva said finding the script/code for that then entering that in your "headinclude" template. Then in the text file if there is one, there should be a line that shows how to make that script work on a site, or in this case, a forum.
Enter that code that is says to get it to work into whatever template you want it to be in.
If that's not right, try heading over to vBulletin.org and typing in the search box "popup thread" and it will find things related to that if any.