Actually, you should be able to RDP into Vista. It won’t be as “pretty” as the new clients, but you can do it.
There is a policy in Vista that is preventing it from happening. I don’t have a vista machine handy, so I can’t give you the exact wording, but I believe it is something along the lines of allow only newer clients to connect or something like that. Basically you need to tell your vista machine to allow it to answer connection requests from previous RDP clients.
You might also run into problems with other policies that prevent you from logging in, but that should get you started.