A mobile app is work in progress[1]. I am currently using the web interface which is quite usable on a mobile phone. Lists work quite nicely, also on smaller screens. I was using Kanboard previously and mobile usage was a bit too complicated.
CalDAV seems to be supported[2]. I have not tried it (yet).
[1]: https://github.com/go-vikunja/app
[2]: https://vikunja.io/docs/caldav/