Occasionally VLC media player generates error message which reads something about not supporting "undf" audio or video format. This might confuse users and some might even try to convert these "undf files". There is no need for any und conversion, simply because the error is generated by lack of proper decoder.
What you need is to check for update for VLC player and / or update your media codecs. which should hopefully fix your issues. No program for undf to avi conversion is needed.Open UNDF file Open AVI file