It's similar to Vue.js version 1, version 2 can be directly embedded in a single html file.
To include Vue.js2 in html file, make sure you have the script file included in the HTML. For example, use the following HTML. It should display the Hello message correctly.
<div id="app">{{message}}</div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.2.0/vue.js"></script>
<script>
new Vue({
el: '#app',
data: {
message: 'Hello Vue.js2'
}
})
</script>
See a jsfiddle demo for this example.