From Oracle FAQ
Oracle Database Vault is a security product that can prevent data stored in the Oracle database from being accessed by privileged database users like DBAs. Application data can be protected with policies that control access based on time of day, IP address, application name, and authentication method, preventing unauthorized ad-hoc access and application by-pass.
Please note that Database Vault and Audit Vault are two completely separate products. Database Vault control the access to your information while Audit Vault let you build a data warehouse to run reports.