Set default value to option select menu

You can just use v-model for selecting a default value on a select box:

Markup:

<div id="app">
  <select v-model="selected">
     <option value="foo">foo</option>
     <option value="bar">Bar</option>
     <option value="baz">Baz</option>
  </select>
</div>

View Model:

new Vue({
  el: "#app",
  data: {
    selected: 'bar'
  }
});

Here’s the JSFiddle: https://jsfiddle.net/Lxfxyqmf/