Edit in JSFiddle

new Vue({
    el: '#container',
    data: {
        message: '#This is a title'
    },
    computed:{
        convertedMessage: function(){
            var text = markdown.toHTML(this.message);
            return text;
        }
    }
});
<div id="container">
    <div>{{{ convertedMessage }}}</div>
    <textarea rows="30" cols="50" v-model="message">

    </textarea>
</div>

              

External resources loaded into this fiddle: