[Kirk Y]

Create a new plugin at hook location "style_fetch" with the following:

switch ($GLOBALS['forumid']){
    case X:
        $styleid = A;
        $vbulletin->options['styleid'] = A;
        break;
    case Y:
        $styleid = B;
        $vbulletin->options['styleid'] = B;
        break;    
    case Z:
        $styleid = C;
        $vbulletin->options['styleid'] = C;
        break;
}

Replace X, Y, and Z with the Forum IDs of the forums you'd like to have specific styles; and replace A, B, and C with the Style ID of the Style you'd like assigned to each respective Forum ID.