If you have multiple NetBackup media servers, you should specify all these media servers in the client. You can specify them during the client configuration or from the host properties of the client. If the client is also a media server, you must explicitly add it as media server.
If you encounter error, check whether the names of all of the media servers among the Enterprise Vault servers are specified in the client configuration. You can obtain the media server name from the job details page in Activity Monitor, of the NetBackup Administration Console.