You can use Safari to download YouTube videos.

  1. Open the page with the movie and press Command-Option-A, which shows the Activity window.
  2. Scroll until you find the YouTube page and click on the arrow to show details about what is being loaded.
  3. Look for an element whose size is over 1MB (most of the time, over 5MB).
  4. Double-click on it (even if it is still loading), and Safari will download it.
  5. When the download is over, navigate to the file in the Finder.  If necessary, add the extension .flv to its name. Now you can play it with VLC.