taxonomy_term_load_children claims to restrict by vid but this is not possible

/**
 * Finds all children of a term ID.
 *
 * @param $tid
 *   A taxonomy term ID.
 * @param $vid
 *   An optional vocabulary ID to restrict the child search.
 *
 * @return
 *   An array of term objects that are the children of the term $tid, or an
 *   empty array when no children exist.
 */
function taxonomy_term_load_children($tid, $vid = NULL) {
  $children = &drupal_static(__FUNCTION__, array());

  if ($tid && !isset($children[$tid])) {
    $tids = \Drupal::entityManager()->getStorage('taxonomy_term')->loadChildren($tid);
    $children[$tid] = entity_load_multiple('taxonomy_term', $tids);
  }

  return isset($children[$tid]) ? $children[$tid] : array();
}

$vid is never used and is non-functional.