Virtual Vault may be enabled automatically

By default, Enterprise Vault automatically enables Virtual Vault for all users who already have Offline Vault.

It is important that you review the Virtual Vault settings to make sure that they do not conflict with your requirements.

For example, the Virtual Vault default settings enable users to move items into Virtual Vault folders to archive them. If you have hidden the Store in Vault option from users you may also want to disable Users can archive items in the Virtual Vault Advanced policy settings.

See the following:

The chapter "Setting up Vault Cache and Virtual Vault" in the Setting up Exchange Server Archiving manual.
The Virtual Vault Best Practice Guide, which is available from http://entsupport.symantec.com/docs/334852.