The problem is the Simmens SpeedTouch 516 is the only device that establishes a PPPoE connection, as it will only allow for one connection. It then has the NAT disabled and is assigned the first IP address in the block. The modem's one and only LAN port is then plugged directly into a switch with 4 of the 5 computers attached to it. The Vonage V-Portal also has its WAN port connected to the switch, with the 5th computer plugged into the V-Portal's LAN port. The more "juicy" details of the configuration can be found in
this forum thread.
If I had the command to use in the SSH terminal to disable the V-Portal's NAT, I could then switch the modem over to bridge mode and wire it directly to the V-Portal's WAN port. From there, the V-Portal would establish the PPPoE session, thus allowing me to use the 76.10.xxx.xxx IP address and assign the 173.xxx.xxx.225 address to the V-Portal's LAN connection, thereby allowing me to plug the V-Portal's LAN port and the computer that once plugged into it into the switch and assign the 173.xxx.xxx.230 IP address to the 5th computer. In this configuration, all devices would be able to be acknowledged as being directly connected, without the need for an additional IP address.
I'm hoping there is a way to do this and that it's just a matter of digging deep enough to do it, but it seems Vonage doesn't seem to have much support for anything to do with the SSH sessions with the V-Portal, which seems a little unusual.