Windows 2008 R2 RRAS VPN/NAT custom setup

I have a Windows 2008 server with VPN/NAT (from RRAS) configured to route all client traffic (e.g. like a proxy does). The server has n public IPs, let`s say IPa, IPb, IPc etc. When a VPN client connects to either one of the server IP addresses, the traffic is normally routed (NAT-ed) with the main server IP (let`s say IPa). I want a setup (done with RRAS, not 3rd party app) so that when client connects (VPNs) to IPb traffic will be routed with IPb, when client connect to IPc traffic will be routed with IPc and so on. This setup has to work with a single NIC.