You can choose to enable Vault Cache automatically for users' mailboxes, or allow users to enable it by running the Vault Cache wizard in Outlook.

The wizard enables Vault Cache for the user's primary mailbox only. If the user has access to other archives, those archives are listed on the Vaults tab in the Vault Cache Properties dialog box in Outlook. The additional archives are not synchronized to the Vault Cache until the user selects them in the dialog box.