Are you talking about your Google account showing up in Settings > Passwords & accounts?
I don't use FairMail, so I checked the
GmailState class in the code. Looks like they add this account to the phone this way in order to store the authentication token.
When you set up your GMail account, did you select "GMail Android", or did you select "GMail OAuth"? From what I can tell, if you select "GMail OAuth", it'll just log in using Vanadium but not add the account to your settings. I think you can clear your Vanadium cache and cookies afterwards to log out of Google in Vanadium, but FairMail should continue to connect. Could be wrong though.
There's a second way that shouldn't require you to add your account in settings, but it's more of a manual setup using IMAP or POP3. To do that, you can refer to the instructions on these two pages: https://github.com/M66B/FairEmail/blob/master/SETUP.md and https://support.google.com/mail/answer/7126229?hl=en
Just note that POP3 can delete your emails from the server, so I'd be aware of that if setting it up this way.