There are the email settings that worked to get vikunja sending emails thru proton bridge.
VIKUNJA_SERVICE_ENABLEEMAILREMINDERS: 1
VIKUNJA_MAILER_ENABLED: 1
VIKUNJA_MAILER_FORCESSL: 0
VIKUNJA_MAILER_AUTHTYPE: plain
VIKUNJA_MAILER_SKIPTLSVERIFY: 1
VIKUNJA_MAILER_HOST: host.docker.internal
VIKUNJA_MAILER_PORT: 1025
VIKUNJA_MAILER_FROMEMAIL: email
VIKUNJA_MAILER_USERNAME: email
VIKUNJA_MAILER_PASSWORD: password