Well, first off, when you drop your VPN connection, do you lose all internet connection? Or is it that the VPN connection is unavailable? It would almost appear that there is something going on in the VPN client config or in the Router config. If all your losing is the VPN I would check those first, if you lose everything it would most likely be in the router config. Check those and see what you can get.
Check your IP address settings on the WLAN interface on both devices. Be sure that the addresses are not the same. It sounds like a duplicate address issue.