If I have already uploaded some music files to my vault, but they are not attached to any playlist. How can I attach the vault copy to a new playlist?

Not very elegant but I went to the file vault, selected the file and then insert. That put the URL on the page being edited. I then went to the Playlist Generator and pasted the above found URL in the appropriate box.

There must be a better way as this is not an elegant solution but it works.

Thanks. I kept playing around and that's exactly what I ended up doing. You're correct that it is not elegant, but it did work.
btw the reason I couldn't upload directly into the widget was the file size is over 10MB.

Thanks FC. Good work around. I will make a note on this issue for future updates.