What version of Videopad is being used?
When you say "converted" to mp4, assume you mean that you EXPORTED it to mp4. That should succeed. Don't 'convert' anything. The VPJ file is created when SAVE PROJECT is used. It's recommended to SAVE PROJECT AS, giving it a unique # or name, so as not to overwrite earlier saves. Using Save Project (no AS) overwrites.
Let's track this down by taking a look at your project. The process is easy, quick and can be private. It won't be shared. Just follow the PORTABLE project instructions here...
http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material link it in a personal message (PM). Click the envelope above. Use FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, upload the resulting folder, and don't forget to get a shareable (non-restricted) link to post. Don''t upload the mp4 export.