<head>
...
<script type="text/javascript" src="//vk.com/js/api/openapi.js?113"></script>
<script type="text/javascript">
VK.init({apiId: ваш_appId, onlyWidgets: true});
</script>
</head>
<body>
...
<div id="vk_like"></div>
<script type="text/javascript">
VK.Widgets.Like("vk_like", {type: "button"});
VK.Observer.subscribe("widgets.like.shared", function f() {
alert('Молодец!');
});
</script>