First, airplane mode doesn't necessarily disable connections, especially if you're on WiFi. It may then be necessary to explicitly disable WiFi.
Second, if all connections are indeed disabled, iOS won't actually let you connect to a VPN -- it will say "no internet connection." It's possible that enabling a VPN, disabling all connections, and then re-enabling connections would allow the VPN to catch any new (re)connections, but I doubt there's any guarantee in a multithreaded environment.
In short, this is hardly a simple workaround and the issue needs to be addressed by Apple directly ASAP.