I have an identical setup. As you are probably aware, the problem is that your VPN router is performing NAT (the router acts as a gateway between your home network and VPN'd network, just like how the router in your DSL/Cable modem acts between you and your ISP. The only way to get this to work is if you have access to your router config. - You'll need to forward the Multiplicity port from the 'WAN' side of your router (WAN being your home network) to your workstations IP address.
Here is the command i use:
ip nat inside source static tcp 10.30.1.68 30564 interface FastEthernet4 30564
10.30.1.68 is the ip address of my workstation.
30564 is the port for Multiplicity
FastEthernet4 is the name of my WAN interface
From here you need to add the IP address that your home network router assigned to the WAN port of your VPN router as a computer in multiplicity.
DHCP for this can be a bear, so I'd recommend either assigning static IPs to your VPN router's WAN port and workstation, or use a static MAC reservation. If DHCP switches the IP addresses on you, you will need to modify the router config.
Here is a script you can run on the router in the event your workstation's IP address changes. - .67 is my old ip address, .68 is the new one.
<ROUTER ENABLE PASSWORD>
no ip nat outside
do clear ip nat translation *
no ip nat inside source static tcp 10.31.1.67 30564 interface FastEthernet4 30564
ip nat inside source static tcp 10.31.1.68 30564 interface FastEthernet4 30564
ip nat outside
Hope this helps someone!