if you already have ProtonMail, then you must be able to use their vpn, as only registered ones can have ProtonMail. Which email did u use to register to Proton? That's what you should use for verification. If you forgot which email you gave them, just key in any email addresses that come to mind, as they will only send the veirification email to the one you registered with. Proton is one of the best, based in Switzerland. Swiss privacy laws prevent them from providing info to anyone, and not even Proton people can look into your info, not even the government, unlike US-based vpn's, if the fbi requests for your browser history, the vpn provider can provide the info. Proton cannot, and do not do that - not in Switzerland