[Plugin: ThreeWP Activity Monitor] Sort Login Statistics within User List

I’d like to sort the latest Login Statistics on the user list. I’ve tried retrieving the information from ThreeWP_Activity_Monitor.php but have been unsuccessful. I’m able to cause the list to sort but the information does not correspond appropriately. I think I may have the meta_key incorrect. Any idea how I could resolve this?

http://i.imgur.com/h3ZWY.png

function user_column_sortable($columns) {
          $custom = array(
      // meta column id => sortby value used in query
		  '3wp_activity_monitor'    => 'latest_login',
          );
      return wp_parse_args($custom, $columns);
}
add_filter( 'manage_users_sortable_columns', 'user_column_sortable' );

function sortable_user_column( $vars ) {
        if ( isset( $vars['orderby'] ) && 'latest_login' == $vars['orderby'] ) {
                $vars = array_merge( $vars, array(
                        'meta_key' => latest_login,
                        'orderby' => 'meta_value'
                ) );
        }
        return $vars;
}
add_filter( 'request', 'sortable_user_column' );

http://wordpress.org/extend/plugins/threewp-activity-monitor/