If you'd rather keep Vikunja as the backend and web UI, it has a CalDAV endpoint, so you can point DAVx5 at it and use Tasks.org or jtx Board on Android for the reminders themselves. That gets you native Android notifications without email, and you keep the web interface for planning. Vikunja's CalDAV side is less mature than its web UI, though, so test whether due dates and reminders come across the way you expect before moving everything over.
For notifications that aren't tied to a task app (say, "remind me when X happens" from a script or Home Assistant), ntfy is the simplest self-hosted push to Android, as mentioned above.
