<div id="demo">
      <div v-for='n in 3'>
        <input v-model="form.parent_id[n]">
      </div>
      <div v-for='n in 3'>
        {{ form.parent_id[n] }}
      </div>
    </div>

    var data = {
        form: {
          parent_id: []
        }
    }

    var demo = new Vue({
        el: '#demo',
        data: data
    })