Rebuild post cache may do it.
Quote:
Rebuild Post Cache:
vBulletin doesn't parse BB code in posts every time they are displayed. Instead the parsed posts are saved in the "postparsed" table. In this way vBulletin reduces the processing time needed to display a post. Update this counter to rebuild the parsed posts. This needs to be done after changing BB code tags, posting allowances (like HTML code), etc.
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Admin CP -> Left column -> Maintenance -> General update tools -> Rebuild post cache.
Or yoursite.com/admincp/misc.php?do=chooser -> Rebuild post cache.