Skype recordings saved with the VodBurner software (.vbc files) can be also exported and saved as MPEG-4 (.mp4 files), which is likely what vbc to mp4 conversion represents in most cases. As far as we know, you can only do that in VodBurner itself, as no other software seems to be capable of converting .vbc recordings.
Open VBC file Open MP4 fileA Skype video recording software