How to get index of selected option with vue.js

Pass $event.target.selectedIndex to your function

Use the @change directive to listen to the change event. Invoke your function and pass the $event or the selected index of the event target $event.target.selectedIndex as a parameter to your function.

<select @change="switchView($event, $event.target.selectedIndex)">

Your method receives the passed parameters.

  methods: {
    switchView: function(event, selectedIndex) {
      console.log(event, selectedIndex);
    }
  }

Full Example https://jsfiddle.net/3p7rhjyw/

<div id="app">
  <select @change="switchView($event, $event.target.selectedIndex)">
  <option v-for="(item, index) in items" v-bind:value="item.title">
    {{ item.title }}
  </option>
</select> 
<p>
{{ selectedIndex }} {{ items[selectedIndex].id }}
</p>
</div>

<script>
new Vue({
  el: "#app",
  data: {
    items: [
      { title: "Learn JavaScript", id: 'A' },
      { title: "Learn Vue", id: 'B' },      
      { title: "Play around in JSFiddle", id: 'C' },
      { title: "Build something awesome", id: 'D' }
    ],
    selectedIndex: 0
  },
  methods: {
    switchView: function(event, selectedIndex) {
      console.log(event, selectedIndex);      
      this.selectedIndex = selectedIndex;
    }
  }
});
</script>

Mozilla documentation about HTMLSelectElement.selectedIndex.

You can use @change on select element and get the index with help of indexOf function. Here is working demo.

See code:

var demo = new Vue({
    el: '#demo',
    data: function(){
        return {
        age: '',
        selectedIndex: '',
        options: [1,2,3,44,55]
      };
    },
    methods: {
      selected: function () {
         this.selectedIndex = this.options.indexOf(this.age)
         alert('this is selected Index ' + this.selectedIndex)
       }
    }   
})