What are some alternatives?
When comparing Vikunja and Todoist, you can also consider the following products
Wekan - The open-source Trello-like kanban (built with Meteor)
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
Google Tasks - to-do list app that integrates with Gmail and Google Calendar
Org mode - Org: an Emacs Mode for Notes, Planning, and Authoring