Authentication is how a user gains access to the Vault Professional repository. Each user who accesses Vault Professional must have (or be assigned) a Vault Professional user account. However, the password required to access Vault Professional can either be the one created and stored in Vault Professional, or one specified by an Active Directory domain account. Active Directory authentication allows the user to change her password in one place and have it apply to both the domain account as well as the Vault Professional account.
When authenticating against the Vault Professional password, merely define the user in Server Settings -> Users -> Add dialog and provide the user’s information and password. The user will use that password when logging into Vault Professional.
When authenticating against Active Directory, two steps are required: Provide the domain against which Vault will search for an account of the same name. This is done in the Admin Web Client through Server Settings ->Advanced Settings in the "Active Directory domain for authentication" field.
Once a domain is specified, an option in the "Active Directory domain for Authentication" drop-down menu in the Add User page will become active, allowing you to require the user to authenticate against their domain account. Note that the domain account needs to have the same user name as the Vault Professional user account. The user will use their domain password when logging in.