How do I hide files, and how secure is the vault?

The vault is not a folder with a passcode drawn in front of it. What goes in is encrypted on the way, and what remains on disk carries a meaningless name; the real names and the folder structure live only in an encrypted catalogue.

What that means in practice

• Its own lock. A 4-6 digit passcode with Face ID or Touch ID and its own auto-lock timer, independent of the app lock.
• Encrypted in chunks, so a film plays straight out of the vault — each piece is decrypted in memory as the player reaches it, and seeking still works.
• Out of sight everywhere else. Library search, Spotlight, Wi-Fi Transfer, device-to-device sending and the share inbox do not see it. In playlists and recently played, a vault item shows as a masked row, and the lock screen shows a generic title while one is playing.
• A decoy vault. The vault can wear a name and icon you choose, opening straight into a harmless set of files, with the real one behind a gesture and a second passcode.
• Nothing is held hostage. Everything inside stays viewable and exportable whatever your subscription state, and one button puts all of it back into ordinary folders.

Files can be moved in from your library with a long press or multi-select, imported from Photos — the copy is verified before you are offered the option to delete the original — or downloaded straight into the vault, where the queue shows a masked row until you open it.

The free version holds 15 items, counted live, so removing one makes room. Beyond that it is FoxDL Pro; this is the one feature with no rewarded-video route, because advertising has no business in a privacy feature.

One limit worth knowing, because no iOS app can close it: screenshots and screen recordings still work while you are looking at something.