EDIT 2

• For all object changes that need reactivity use Vue.set(object, prop, value)
• For array mutations, you can look at the currently supported list here

EDIT 1

For vuex you will want to do Vue.set(state.object, key, value)

Original

So just for others who come to this question. It appears at some point in Vue 2.* they removed this.items.$set(index, val) in favor of this.$set(this.items, index, val).

Splice is still available and here is a link to array mutation methods available in vue link.