Vue is not defined

jsBin demo

1. You missed the order, first goes:

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/1.0.18/vue.min.js"></script>

and then:

<script>
    var demo = new Vue({
        el: '#demo',
        data: {
            message: 'Hello Vue.js!'
        }
    });
</script>

2. And type="JavaScript" should be type="text/javascript" (or rather nothing at all)

Sometimes the problem may be if you import that like this:

const Vue = window.vue;

this may overwrite the original Vue reference.