ISP already knows your IP, they allot it after all.
You are misunderstanding what VPN does. When connected to a VPN, your web requests look like this (simplified):
You > ISP > VPN > Internet
So the ISP can only see you connect to the VPN (and not even that if you use stealth and your ISP doesn’t so DPI).
So in ProtonMail case, your flow would be:
You > ISP > ProtonVPN > ProtonMail
The ISP would know you connected to ProtonVPN. ProtonMail will know ProtonVPN IP connected to it.
So your ISP knows your IP and your connection to VPN, but not your connection to Mail, and Mail knows your ProtonVPN IP but not your personal IP. So Mail can only log your ProtonVPN IP, and your ISP can only log your personal IP and connection to ProtonVPN. Since ProtonVPM cannot be forced to log your IP or activity, there is no way to correlate the activities on Mail and VPN to trace them back to you.