[kh99]

I don't know if there's any existing mod, but if instead of specific forums you enable it for specific user groups (like only for admins and moderators), then you could use this plugin:

PHP Code:

if (is_member_of($vbulletin->userinfo, 5, 6, 7))
    $vbulletin->options['threadpreview'] = 150; 


using hook location global_start (of course you can change 150 to whatever length you want).

You might be able to do the same thing and check forum ids instead of usergroups, but I haven't tried it.