Need help with complex VLAN setup

I am trying to set up the following and need help with this.
Host is Mac OS X 10.5.2 Leopard, one guest is Windows XP Pro (SP2) and second guest is RedHat Enterprise 4 running Oracle 10 and various custom servers. On Windows we use proprietary development tools in client/server mode against the Oracle server. Some processes on the Oracle server communicate with servers in the outside world. This setup functions well using shared networking thus forming a virtual LAN inside my Mac. Some port forwarding to the linux box allows access from the host system (Mac). Now I have to access yet another remote server that unfortunately does not send its response back over the same network channel but wants to send it back to a dedicated port. But my RedHat server sits behind NAT (I can open the port for forwarding) but how do I get the traffic coming to my Mac for this dedicated port to the guest adaptor? And what address must the remote server send its reply to? Is this possible at all? If need be I can run the whole thing in bridged mode and use static IP addresses but is it then possible for the two guests to talks to each other? Can the host talk to the guests? The whole thing runs on a Mac Book Pro that is not always connected to the same LAN. When I am working off-site I need to establish a VPN connection to the customers site. And for some tasks I do not need the outside servers. In this case I should be able to work without outside network connection at all e.g. commuting on the train.
Any idea how to implement such requirements?
Thanks for your help

 

You can think about using Bridged Networking, or configuring the ipfw and natd daemons.