Set a folder to be shared on the Vista (don't forget to add the user of the Vista machine you want to share folders) and access it trough the Leopard.
Control + Click (Right click) on Finder > Connect to Server.
Write there the IP address of your Vista. It will ask the login name and password of the user your previous set access.
PS: It's not the dynamic IP address, it's the IP assigned to the router.
Start > Execute > "cmd.exe"
Type ipconfig and check the IP address. If your router IP is 10.1.1.1 then your IP will be something like 10.1.1.X.
