Edit in JSFiddle

window.eventBus = new Vue();

new Vue({
  el: '#app',
  data: {
    simpleString: ''
  },
  created () {
    var self = this;
    eventBus.$on('stringUpdated', function(string){
      self.simpleString = string;
    })
  } 
});

new Vue({
  el: '#app2',
  data: {
    simpleString: ''
  },
  created: function () {
    self = this;
    eventBus.$on('stringUpdated', function(string){
      self.simpleString = string;
    })
  } 
});

eventBus.$emit('stringUpdated', 'hello just a string');
<script src='https://unpkg.com/vue@2.4.4/dist/vue.js'></script>

<div id='app'>
  {{simpleString}}
</div>
<hr />
<div id='app2'>
  {{simpleString}}
</div>
<hr />
<input onchange="eventBus.$emit('stringUpdated', this.value)"/>