$ not defined using jQuery in WordPress

You can wrap your javascript inside a self-invoking function, then pass jQuery as an argument to it, using $ as the local variable name. For example:

(function($) {
  $(document).ready(function(){
    $("ul.vimeo_desc_feed li a").click(function(){
      alert($(this).attr('href'));
      return false;
    })
 });
}(jQuery));

should work as intended.

If I remember correctly the WP-supplied version of jQuery (the one you get if you wp_enqueue_script('jquery')) puts jQuery in no-conflict immediately, causing $ to be undefined.