> it would be great if the app offered an option to hide its files from other apps by locating them in the app's private storage area

For what it's worth, you can do just that now. Not sure when it was introduced, but on the current Android version, you are prompted to select either "Device storage" or "App storage" when creating a new vault.

You can also change the location of existing vaults from app to device and vice-versa.

With that now available, the one thing missing for me is a biometric lock option. Or even any kind of locking at all. There's a community plugin for that, but it's quite unstable and no longer maintained.