Airport slows down when anybody uses VPN through it
It's more than likely the bandwidth used by your VPN rather than the Airport itself.

My work VPN connection with a corporate laptop used to use a lot of bandwidth, I'd notice other stuff slowing down considerably. Maintaining links to the domain servers, a VOIP client, and all of the other things installed on there used a lot of resources.

When I stopped using a corporate laptop and connected my personal laptop through a web based SSL VPN with terminal services instead, things improved a lot as it was only basically screen sharing rather than uploading and downloading actual files and data.

Unless you can set-up a proxy and monitor the usage of each device, you're not really going to find out how much bandwidth each device is using, but it's unlikely to be the VPN itself, or the Airport at fault, just the bandwidth used by what's being transferred over the VPN.