There is a case of an activist being identified by ip address while using Proton Mail. While the content of the email was never exposed, Proton released a statement that if the user in question had been using Proton VPN as well as encrypted mail there would have been no possible way to identify the activist by the French government. Powerful adversary and the encrypted data remained locked. I was impressed.
Discussion
That is indeed impressive. I actually avoided using both Proton Mail and Proton VPN together because I didnt want all my eggs in one basket but this is making me reconsider this combo.
You can also run the VPN on a wired router with a software kill switch, then run a Tor proxy on your computer. While the VPN will know your identity and encrypt the traffic from the router to your ISP, the traffic from your computer will be Tor encrypted and unreadable to your VPN provider. Some software can route traffic around computer based VPN software and proxies, but a router based VPN eliminates that potential for an IP leak. It’s also possible to access Proton through Tor and set up a free account for anonymous emailing.