I guess you could just keep the registry data for that VPN? Not really sure if your login details are stored there.
Also there are other ways to get rid of junk on your system without formatting the whole drive. I can recommend BCU (Bulk Crap Uninstaller) for deleting old shit without any thing left. It has a build in registry back up function and the ability to clean up leftover's after uninstalling. Pretty neat tool.