I was looking for upload/download speed and latency. I don't know what operating system or VPN software you're using, but what I was planning to do:
1) Ping through the VPN tunnel (the software I use, OpenVPN, shows up as a network interface like a normal network card),
2) Download a medium-size (less than 15MB) once or twice file to see what the download rate is, and
3) Figure out some way to test interactivity--maybe an SSH connection over the VPN to see if it's any worse than SSH over satellite (which is pretty hilariously bad already).
Actually I can test another type of "VPN" right now: SSH allows a user to set up an encrypted tunnel and use it as a SOCKS proxy. So I'll try downloading a few files through the SSH tunnel with Firefox. :)