Edit in JSFiddle

const globalData = { 1: 'a' };

var vm = new Vue({
  el: '#app',
  data: {
    globalData,
    privateData: Object.assign({}, globalData),
  },
  watch: {
    globalData: {
      handler(val) {
        this.privateData = Object.assign({}, this.globalData);
      },
      deep: true
    }
  }
})
<div id="app">
  private: {{ privateData }}
  <select v-model="privateData[1]">
    <option value="a">A</option>
    <option value="b">B</option>
    <option value="c">C</option>
  </select>
  <br> global: {{ globalData }}
  <select v-model="globalData[1]">
    <option value="a">A</option>
    <option value="b">B</option>
    <option value="c">C</option>
  </select>
  <br>
</div>