The only vpj to mp4 conversion we know of, is export of .vpj projects from VideoPad to MP4 format, typically so the .mp4 file can be uploaded to YouTube.
To create a video in VideoPad, click the Export Video button in the Home tab toolbar, and choose MP4 as output format to save it.
This is doable only if you also have access to the original multimedia data referenced in .vpj projects as these are just sessions to store work in progress. Without the multimedia data it will be impossible to reconstruct the video and export it and a lone .vpj file is more or less useless.
Updated: April 13, 2017