Public WiFi - credentials compromised before VPN is started

[Scooby-2]

I am trying to connect to my home OpenVPN server from an Internet Cafe via WiFi using my Android phone. I presume that, in order to get new mail/WhatsApp messages my IDs and passwords are sent. However, I get the notifications/emails before the VPN connection is established (as it happens I cannot get it to connect, but that's a different issue). I have googled "disable auto sign on Android" but it is clear that this is something that can't be done, certainly not for Gmail.

All I want to do is stop my mobile Apps accessing the web the instant any old network is available, and have them wait until the VPN connection is established. How can I do this?