If you no longer require the former employee data, you can delete the mailbox archive in the VaultAdministration Console, which automatically removes the index associated with that archive.
Another option is to simply move the IndexLocation and it's content to a drive with more disk space and then create a new Index Location somewhere else.
A closed index location can still be written to and update existing indexes in it, however, new indexes will get created the new, open index location
Document ID: 273141
http://support.veritas.com/docs/273141
How to move an Enterprise Vault index on the same Enterprise Vault server