I would try to get the vaults in the center of the loads they serve (not on one end). That helps your voltage drop/copper problem. I have seen the drops run in a string (big copper, stepping down) or radially with smaller copper depending on the layout. I have seen a main and 6 breakers or exploiting the "outside tap" rule with just a main Polaris blocks and the breakers on the pedestals (watch the engineering on this one).
