The last few times I have tried, have not been able to connect to the paid inflight WiFi using a VPN.
Wait- did you mean "Connect to a VPN while using the inflight WiFi", or did you actually mean "Connect to the paid WiFi
while using a VPN"? If the latter, that's not going to work. Disconnect from your VPN first, then pay (or authenticate if you get it for free), then re-connect to your VPN. It's no different from say, a hotel's or cafe's Login Portal page where you need a working connection first before you can pass any traffic.