I feel like an idiot for having to ask but why doesn't this work? The default fields work fine but I can't get the additional fields to work.
Code:
$stc_user_info = fetch_userinfo($currentuserid);
$additional_fields = array(primaryclanid,lastlogoff);
$steam_info = fetch_steam_info(get_user_steamid($stc_user_info), $vbulletin->options['stc_apikey'], $additional_fields);
$steamavatar = $steam_info['avatarfull'];
$steamusername = $steam_info['personaname'];
$visibility = $steam_info['communityvisibilitystate'];
$clanid = $steam_info['primaryclanid'];
$lastlogoff = $steam_info['lastlogoff'];
Thanks for any help - I know it's not really your job to sort out issues with customisation.