in the admin cp -> usergroups -> usergroup managed.
choose and edit the usergroup you wish to change.
in the blocks add what you would like them to be :
Username HTML Markup
(Specify an opening and closing HTML tag with which to display the names of users in this group. This applies to names that appear on Logged-in Users, Who's Online, Users Browsing Forum, Users Browsing Thread, Posts, Member List etc.)
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think there is a mod for this at vBulletin.org.. .