Edit in JSFiddle

  const vm = new Vue({
    el: "#app",
    data: {a: false},
  })
<div id="app">
  <div class="form-horizontal">
    <div class="form-group">
      <div class="col-md-offset-2 col-md-10">
        <label class="checkbox-inline">
<input type="checkbox" v-model="a">承認</input></label>
      </div>
    </div>
    <div class="form-group">
      <div class="col-md-offset-2 col-md-10">
        <div class="btn btn-primary" :disabled="!a">
          送信
        </div>
      </div>
    </div>
  </div>
</div>