Hi
You need the .vpj file. Without it your project is lost in the event that an autosave was also lost. Using "Save" or "Save as" regularly will ensure that in the event of a crash you will still be able to get the project back....at least to the point where the save was made.
The ,vpj file is basically a text file containing all the instructions that enable VP to reconstruct your project. names of video clips used, their locations on your PC, details of the various edits you made, effects and transitions etc.
The cache file on the other hand holds copies of clips and thumbnails generated by VP whilst you were working on the project as well as any previous projects you may have worked on if the cache was not cleared at some previous point. (Tools/Options/Disk)
The .vpj file will recreate your project as it accesses the clips from the cache making the loading faster as thumbnails etc. are already present and applies the editing steps it has stored..
Even if the cache has been totally cleared the .vpj file will still be able to recreate your project as it contains all the details of where your clips are stored, but it will take appreciably longer. It needs to find your clips from your PC now. load them, recreate the thumbnails and then apply all the editing steps you made. If you have deleted any clips from your PC or moved them to a different location VP will let you know and prompt you to manually find them.
So, as mentioned at the start you should regularly save your work.
Nat