Is there a twitter update add-on for vB 3.8.4? Nevermind, found one. http://www.vbulletin.org/forum/showthread.php?t=204694
If all youre wanting is the ability to tweet your posts I think this may be good for you: Submit to Twitter, Facebook and Digg - vBulletin.org Forum Just use the Twitter bit and leave the rest if you only want Twitter
I had problems with the one you actually linked to, so an alternative (if you ever do run into them) is Tweet Poster which handles things much more efficiently and integrates bit.ly nicely into things!