I don't know how to do it in VB, but this is how it works.

For example, you have this url to a Youtube video: http://www.youtube.com/watch?v=TJhP6pyVu20

Change the url to this: http://www.youtube.com/v/TJhP6pyVu20

When you paste the above (second) url into your browser's address bar and press enter you'll get this url in your browser's address bar: http://www.youtube.com/swf/cps.swf?v...rel=1&border=0

Extract the "video_id=" parameter and the "t=" parameter from that url.
Then add them to the following url: http://www.youtube.com/get_video.php?

And you'll get the the url to the file:

http://www.youtube.com/get_video.php...-yUTlV1RHWt8ua

Save the file with a ".flv" extension.