If you've forgotten the secure vault passphrase, then recovery will require the destructive operation of clearing the secure vault. After clearing the secure vault a new, empty one will be available and you will need to readd the lost components, namely the encryption key and cloud credentials. There is no way to recover the contents of a secure vault unless the passphrase is known. If you don't have the encryption key saved as part of a configuration backup of AltaVault, then you may have to start over from a backup perspective, writing data to a new cloud bucket (or clearing the contents of the existing bucket).
The process to clear the secure vault is by issuing these CLI commands to AltaVault:
# config t
(config) # secure-vault clear
(the system will automatically reboot)
When the system has booted, it will be missing the cloud credentials and encryption key. These can be re-entered under Configure > Storage > Cloud Settings, or imported from a previously saved configuration archive using Configure > Setup Wizard.