Vault is a privacy application developed by NetQin Mobile. This application allows you to hide files and information on your phone from others.
Once you've installed Vault, you'll be able to create a password that hides all of the content you've chosen within Vault. From there, you can select files and applications that can only be opened with this password.
Users who choose to subscribe to the Premium plan get some cool features like being able to hide the Vault application completely from the home screen of your phone so nobody knows you are hiding something. For those that think you have Vault installed, you can even create a fake Vault in which you insert files that you don't care if they get shown on your phone. Premium users also have the option of backing up any files to the 5GB of cloud storage provided to them.
Overall, Vault is a very unique and useful privacy application that is worth it for anyone looking to hide files on their phone.