SOURCE

new Vue({
  el: '#with-mode-demo',
  data: {
    on: false
  }
})

<div id="with-mode-demo" class="demo">
  <transition name="with-mode-fade" mode="out-in">
    <button v-if="on" key="on" @click="on = false">
      on
    </button>
    <button v-else="" key="off" @click="on = true">
      off
    </button>
  </transition>
</div>

.with-mode-fade-enter-active, .with-mode-fade-leave-active {
  transition: opacity .5s
}
.with-mode-fade-enter, .with-mode-fade-leave-active {
  opacity: 0
}
</style>