See main post for download.
Changelog, 21 Feb 2013:
- added 'URL' source to Capture tab: can now capture/transcode URLs straight to chosen file format
- updated help info in main GUI
- updated tooltip info in Capture tab
These changes mean you can now convert a youtube URL straight to an mp3 file.

Just put the youtube URL in the URL field of the Streams tab, and then go to the Capture tab, click on the 'URL' source, choose the conversion settings, and click the Capture button --> VLC-GTK will then convert the youtube video in the URL straight to a local mp4/flv/mp3/etc file...
I used it to go through a list of youtube URLs of some good songs, and give me the mp3 file of each song
