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On the old phone: Settings → Backup → Export encrypted backup, choose a passphrase, and save the file somewhere you can reach from the new phone (Files/iCloud Drive, AirDrop). On the new phone: Settings → Backup → Restore from backup, pick the file, enter the passphrase.

Codes are not moved automatically by iCloud or a device transfer — that is deliberate. Make the backup before you wipe the old phone.

The secrets were only on that phone and cannot be recovered by anyone, including us. Use the recovery codes each service gave you when you set up two-factor, or contact each service's support. Then make a backup on the new phone.

The file cannot be opened without it — there is no reset, because nothing about it is stored anywhere. If the app is still installed, make a new export with a passphrase you'll remember.

Almost always the phone's clock. Codes are computed from the current time, and a clock more than a few seconds off produces the wrong code. Check Settings → General → Date & Time → Set Automatically is on.

If only one account misbehaves, open it and check the digits, algorithm and period match what the service expects — nearly everything uses SHA1, 6 digits and 30 seconds.

Yes. The file is a documented format that a short Python script can decrypt — see Backup format. Inside are standard otpauth:// links that any authenticator can import.

Because syncing means your secrets exist on a server, and not having that is the point of this app. Use an encrypted backup to move accounts between devices deliberately.

Vault2FA uses whatever the device has: Face ID, Touch ID, or the passcode. If Face ID is set up but not offered for Vault2FA, check Settings → Face ID & Passcode → Other Apps.