Sort WP_User_Query by meta_key value with pre_user_query

As written, this is unlikely to do anything. If you add a couple of var_dumps like this:

function sort_by_member_number( $vars ) {
  var_dump($vars);
  if ( isset( $vars->query_vars['orderby'] ) && 'member-number' == $vars->query_vars['orderby'] ) {
      $vars = array_merge(
          $vars->query_vars, array(
              'meta_key' => 'arcc_member_number'
          )
      );
  }
  var_dump($vars);
  return $vars;
}
add_filter( 'pre_user_query', 'sort_by_member_number' );

You will notice that nothing has changed before and after your code, and you won’t find your arrc_member_number value in there anywhere. And, if you look at that dumped data, you will notice that the query has already been constructed (sample query only):

["query_fields"]=>
  string(30) "SQL_CALC_FOUND_ROWS wp_users.*"
["query_from"]=>
  string(13) "FROM wp_users"
["query_where"]=>
  string(9) "WHERE 1=1"
["query_orderby"]=>
  string(28) "ORDER BY user_registered ASC"
["query_limit"]=>
  NULL

And if you look in the source for where the query actually runs, those are the properties used.

In other words, pre_user_query does not work like pre_get_posts. You will need to actually parse that query_* values, which can be tricky and means you need to be good with SQL. If you can write your user query to be something like this…

$user_query = new WP_User_Query( 
  array ( 
    'orderby' => 'arcc_member_number',
    'meta_query' => array(
        array(
            'key'     => 'arcc_member_number',
            'compare' => 'EXISTS'
        ),
    )
  ) 
);

… it will get you started but you still need to alter query_orderby because there are hard-coded limits to what you can order by.

function sort_by_member_number( $vars ) {
  if ( isset( $vars->query_vars['orderby'] ) && 'arcc_member_number' == $vars->query_vars['orderby'] ) {
    $vars->query_orderby = 'ORDER BY arcc_member_number ASC';
  }
  return $vars;
}
add_filter( 'pre_user_query', 'sort_by_member_number' );

You are only gong to get users that have the arcc_member_number meta_key with that. If you need all users, just sorted by that key you need still more query manipulation.