Keep two spaces distinct
A decoy vault gives you another collection to open without bringing your main collection into view. Use it for ordinary photos you are comfortable displaying.
Choose its contents deliberately
Treat the decoy as a separate space. Add the photos you want it to contain and check the collection before relying on it in a situation where privacy matters.
Understand what it does
A decoy changes what appears in the app. It is not a promise that other vaults cannot be discovered through device inspection, and it does not replace protecting your phone and access credentials.