Edit in JSFiddle

      var app = new Vue({
        el: '#app',
        methods:{
          button: function(event){
            alert('hello vue!')
              
          }
        }
      })

              
<!DOCTYPE html>
<html lang="ja">
<head>
    <meta charset='utf-8'>
    <title>Vue App</title>
    <link href="main.css" rel="stylesheet">
</head>
<body>
    <div id="app">
        <button v-on:click="button">button</button>
    </div>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.5.13/dist/vue.js"></script>
    <script></script>
</body>
</html>