Wordpress - $ 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.

You're almost there!

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

});

You have to pass a reference to jQuery as the $ function into your method or it won't work. If you just place a $ inside the first function() call as I did above, things will be working just fine.

Passing a function to jQuery is shorthand for $(document).ready(...) then by placing $ as the first parameter of your callback, you create an alias for jQuery within that callback:

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

You can see the documentation for this here.