A simple way, without template/file edits is to use the integrated censorship system of vBulletin. Go to ADMIN CP > vBulletin Options > Censorship Options
.. and add to the field "Character to Replace Censored Words" an "[at]" (without "").
After remove all the content of the field "Censored Words" and add only a "@" (without "").
It's ok if you don't use this system on your site in a serious way (for the censroship of other "bad" words).
Sorry for my bad English.