Great work! Here's another way to eliminate the method but keep the button and click binding:

<button @click="task.completed = !task.completed">Toggle</button>

Here's the fiddle:

https://jsfiddle.net/harps116/2hmu4yyh/2/

This technique is pretty handy for toggle scenarios. Of course you could abstract the logic into a method for reuse and have a global toggle utility method which might be useful.