Just make sure you don't lose the passkey to log into your email.

Isn't it the same as my password manager's vault? I only remember my master password so if that vault is lost I can't even log in to my email

Kind of, but I have some of my most important passwords and account recovery codes duplicated on paper in a secure place. If there was ever a service that only allowed passkey login (do those exist?), you can't print those out.

I'd still prefer password+2fa+backup codes for email.