[Scanu]

Do you want to do this everywhere in the forum? If so I can't test it but i think you can do this by creating a field "Display name" then create a plugin with global_start hook with this content

PHP Code:

if ($vbulletin->userinfo['fieldX'])  //Check if you put the name to display
$vbulletin->userinfo['username'] = $vbulletin->userinfo['fieldX']; 


Where fieldx is the id of the field you just made, but i don't think it will works everywhere, for get it working in all the pages i think you can figure it out by yourself