Edit in JSFiddle

new Vue({
  el: '#app',
  data: {
    fisrtName: 'Vinicius',
    lastName: 'Reis',
  },
  computed: {
    fullName() {
      return `${this.fisrtName} ${this.lastName}`;
    }
  }
});

<div class="container" id="app">
  <form>
    <div class="form-group">
      <label>First Name</label>
      <input type="text" class="form-control" placeholder="First Name" v-model="fisrtName">
    </div>
    <div class="form-group">
      <label>Last Name</label>
      <input type="text" class="form-control" placeholder="Last Name" v-model="lastName">
    </div>

    <div class="alert alert-info">
      {{ fullName }}
    </div>
  </form>
</div>