SOURCE

var Main = {
    data () {
      return {
        radio: '1',
        img: ''
      };
    },
    methods: {
      change (el) {
        const file = this.$refs.file.files[0]
        const reader = new FileReader()
        reader.readAsDataURL(file)
        reader.onload = () => {
            const _base64 = reader.result
            this.img = _base64
        }
    	}
    }
  }
var Ctor = Vue.extend(Main)
new Ctor().$mount('#app')

<script src="//unpkg.com/vue/dist/vue.js"></script>
<div id="app">
<template>
  <div>
    <input ref="file" type="file" @change="change">
    <img v-if="img" :src="img" width="300">
  </div>
</template>
</div>