This setup would be hooking up the old one to one of the two, not hooking all 3 together.
Ah... Why not? It would be trivial to do so. Then you just make sure to set up account permissions on your shared resources to prevent the others from accessing it.
If you only want to network two computers together, the simplest way would be to plug them directly in to each other - provided you have two NICs on your new Vista machine. Then set up your XP machine and the secondary ethernet port on your Vista machine with static IP addresses. These IPs need to be on the same subnet as each other but different than your main network so your Vista Machine doesn't get confused.
For example:
Vista NIC 1 - Gets IP via DHCP from your router (or static if you swing that way)
IP = 192.168.1.42
Subnet = 255.255.255.0
Gateway (your router) = 192.168.1.1
Vista NIC 2 - Manually set the static IP so this machine can talk to the other XP box.
IP = 192.168.2.1 (or any other variant of the private IP space so long as it isn't 192.168.1.x)
Subnet = 255.255.255.0
Gateway (itself) = 192.168.2.1
XP NIC - Manually set the IP so this machine can talk to the Vista box it is hardwired to.
IP = 192.168.2.2 (must be in the same subnet as the Vista NIC 2 it is connected to)
Subnet = 255.255.255.0
Gateway (Vista NIC 2) = 192.168.2.1
I hope that makes sense to you. Let me know if you have any questions.