Mavaul vs the Hidden album
Hidden is still Photos
The Hidden album is a folder in Photos. You need Face ID to open it, which is better than the old scroll-up trick. The photos are still in Photos. They can still sync to iCloud. Search and Memories are the reason people move things out, and Apple has changed how Hidden works more than once.
Mavaul is a different app
The photos aren't in your library. The lock is a passcode you set for the vault, not the one that unlocks the phone. AES-256 on the device. You can swap the icon for a calculator, a cat, or a bottle of hot sauce so the home screen doesn't say vault.
When Hidden is enough
Hidden is fine for "don't show this while I'm flipping through vacation photos." It is not a second encrypted store. If that's what you want, use a vault. Privacy says what leaves the phone. Apple explains the Hidden album in Hide photos with the Hidden album. By default, nothing does.