<div id="app">
  <input v-model="formula" placeholder="式を入力して下さい">
  = {{response}}
</div>

var app = new Vue({
  el: '#app',
  data: {
    formula: ''
  },
  computed: {
    response: function() {
      return this.formula ? eval(this.formula) : '答えを出力します';
    }
  }
})