I'm not sure if it's possible, but I do not know any possibility of having one "card port" belonging to two or more VPN's (If it is, you will be sharing bandwidth and that's not what you want)... So, you'll need one port for each VPN and then, of course, one interface for each vpn... At least if "port" means the place where you plug the wire...
You don't need to buy more NICs if you have free ports.
Regards.