You set SSO_SCAN_ABILITY priority in vm.conf. If you want your one of the media server to be the scan host you can set:
SSO_SCAN_ABILITY = 9
on that particular media server and on all the other media server set:
SSO_SCAN_ABILITY =0
But doing so if that media server goes down, there can be problem.
Hope it might help.
Bhaskar