Plesk licenses per server. Per a conversation with one of their sales reps (Paul Burdick to be exact) the license key opens the software up for the entire server, so you can install it as many times on that server as you like as long as you do not exceed the number of domains that are licensed. So, an unlimited license permits unlimited use on that server even within individual VPS'.
Not sure about cpanel, but judgin by the way it does the remote license authentication I think there may be troubles.
I was looking through the CPanel change logs and saw some stuff in there about testing 'vservers'. SO... it is probably a long shot, but I would imagine cpanel will be integrating this feature into the control panel in the future. (Now, this could be way down the road, so you may or may not want to wait on it.)