Edit in JSFiddle

<div id="editor">
  <textarea v-model="input" debounce="300"></textarea>
  <div v-html="input | marked"></div>
</div>
new Vue({
  el: '#editor',
  data: {
    input: '# hello'
  },
  filters: {
    marked: marked
  }
})
html, body, #editor {
  margin: 0;
  height: 100%;
  font-family: 'Helvetica Neue', Arial, sans-serif;
  color: #333;
}

textarea, #editor div {
  display: inline-block;
  width: 49%;
  height: 100%;
  vertical-align: top;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  padding: 0 20px;
}

textarea {
  border: none;
  border-right: 1px solid #ccc;
  resize: none;
  outline: none;
  background-color: #f6f6f6;
  font-size: 14px;
  font-family: 'Monaco', courier, monospace;
  padding: 20px;
}

code {
  color: #f66;
}