For running a FSRM walk on the CIFS path, both the Agent & CIFS server(in this case vFiler) must be in the same domain.
Credentials of the user with access on the CIFS path must be provided to the Host Agent.
Host Agent will use this user's crendtials to do a walk on the CIFS path.
Home -> SRM Summary -> <click on the Host Agent from the list>
& this will open the Host Agent Details page.
On this page, click on 'Edit Settings' (near left bottom), and then provide the CIFS account & password.
Agent CIFS Account - User with access to the CIFS path.
Agent CIFS Password - Password of the above user
>Can there be a different UNC authentication for each Host Agent?
Yes, see above steps for providing the authentication.