An mp4 file will not suffice. The VPJ file is a control file and doesn't contain source files.- it points to them, and stores all editing decisions.
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. Then a project can be restored to the state it was in when saved.
If it cannot find the files then they have either been deleted or moved. It will prompt to resolve them. If they are still on the PC, point each 'can't find' prompt to the correct file location. If they aren't on the PC, locate them and put them back - preferably in their original location path. Or try placing them in the same folder as their VPJ file.
A good way to store all contents of a project - source files, layout, transitions, effects, and the VPJ - is to click on Menu (top-left), click FILE, then choose BACK UP PROJECT FILES TO A FOLDER.
This is a portable folder that can be used to restore the project on any PC that has a VP release installed that is reasonably close to the one that created it..