I think the problem might be that $foruminfo isn't registered to the ad templates (as you mentioned in the other thread). To do that you'd use a plugin on hook parse_templates and code like:
Code:
global $foruminfo;
vB_Template::preRegister('template_name', array('hideads' => $foruminfo['hideads']));
(of course you'd use the actual termplate name). Then in the template you'd check $hideads.