You can only use Mobile Search with a single-site configuration of Enterprise Vault. An Enterprise Vault site comprises one or more Enterprise Vault servers that are configured to archive items from specified target servers.
Note the following:
Mobile Search requires access to the domain controller and Enterprise Vault server(s). We recommend that in a production environment you should deploy it on the intranet behind a firewall. Mobile Search should be made available on the Internet through a reverse proxy server in the DMZ. However, a reverse proxy server in the DMZ is not mandatory, and Mobile Search can be installed without it.
We recommend that in a production environment you should install Mobile Search on a separate server from Enterprise Vault and certain other applications.
See Prerequisites for Enterprise Vault Mobile Search in a production environment.