SOURCE

let app = new Vue({
  el: '#app',
  data: {
  	first: '',
    last: ''
  },
  computed: {
  	fullname: function() {
    	return this.first + " " + this.last;
    }
  }
});

<div id="app">
  <h1>Hello {{ fullname }}</h1>
  <p>First Name: {{ first }}</p>
  <p>Last Name: {{ last }}</p>
  First Name:<input type="text" v-model="first">
  Last Name:<input type="text" v-model="last">
</div>