If a developer abandons their app, there’s nothing really you can do. In the future, never trust any data you value to a single source for storage. Always make sure you have some independent backup of that data (export the images to an encrypted folder on your laptop or desktop, or somewhere like Dropbox or Google Photos).
The reality of software licensing is that EULA‘s allows a developer to walk away from their software whenever they chose to. So while using something like Photovault may be fine, just make sure you have your data itself backed up somewhere independent of that app. That goes for any proprietary encrypted storage, like password managers (many of which do offer the ability to export the database as CSU text, which can again be saved in an encrypted folder somewhere).