On formatting your phone where does all your media go that was previously placed in the vault?
If this is the app that you used, then according to their FAQ (found in the app details in play store) the images are not stored online, and therefore were deleted during your factory reset. there is no easy way to restore them.


  Are my hidden files stored online? 
  No. Your files are stored only on
  your device, so please make sure to backup all your hidden files
  before transferring to new device or factory reset