The hiding the links from visitors but not search engines part doesn't sound like too good of an idea.
But if someone made a script that constantly checked (once every 6 hours or so) the vBulletin Link Directory like this category for computer games then it would be doable,
http://www.vbulletin.com/links.php?l...computer_games
However depending on the site, make sure you pick the right category from here
http://www.vbulletin.com/links.php