The VPN is also an option, but we need to test.
I tried your suggestion, but the issue is when you install update manger (on the same box as vcenter, as we do) after setting the IP address as you suggested in your last post, during the install of update manger it specifically asks in the second dialogue box, for the IP address as described::
"enter the connection information for the update manger"
" specify how this vmware vshere update manger should be identified on the network. please make sure this IP address or hostname can be accessed from both vcenter server and hosts"
So its at this point of the install wizard that the only IP's we get to select in a dropdown are the private IP address of vcenter, or its hostname. It looks like the installer is simply only letting you put the IP addresses that you would have in the hosts file of the vcenter server.