You don't even have to have unused physical extension ports. Valid extension numbers for ACS R5 are 10 to 57. Just give them Auto VMS Cover (#310), and set the VMS Cover Rings to 1 (#321). Calls to those extensions will go to VM after 1 ring.
You say your VM is R3.0. Is this Partner Mail VS R3.0 in a slot in the carrier, or Partner Mail R3.0 standalone on the wall? I ask because on the VS mail, you can assign the extensions used for the VM ports as guest mailboxes as well, and the VM will handle everything properly. On the standalone units, the calls will be handled properly, but when the Message Lights are lit for mailboxes numbered the same as the VM ports, it causes false calls to the operator.