Vault is a system that allows a group of people to store and access items in a secure fashion on your Drupal site. Data that is pushed to the server is encrypted with a shared key and is decrypted on the fly when you "open" a vault. This provides a small a window of opportunity as possible for someone to break into your server and steal vault credentials.
Vault is a work in progress. Currently it can only encrypt text data, but the hope is that it will be able to encrypt a wide variety of data types in the future.
There will not be any official releases of this module until it has had a full security audit and I can confirm that it has no serious security vulnerabilities. Until that time use at your own risk.