modules/taxonomy/views_handler_argument_vocabulary_vid.inc

<?php

class views_handler_argument_vocabulary_vid extends views_handler_argument_numeric {
  function title() {
    $title = db_query("SELECT v.name FROM {taxonomy_vocabulary} v WHERE v.vid = :vid", array(':vid' => $this->argument))->fetchField();

    if (empty($title)) {
      return t('No vocabulary');
    }

    return check_plain($title);
  }
}