Edit in JSFiddle

    var app = new Vue({
        el: '#app',
        data: {
            message: ''
        }
    })
textarea {
  border: 2px solid #777777;
  border-radius: 3px;
  width: 98%;
  height: 70px;
  font-size: 15px;
}

textarea:focus, input:focus, input[type]:focus, .uneditable-input:focus {
  outline: 0;
  box-shadow: none;
  border: 2px solid #4caf50;
}

h4 {
  margin-bottom: 0;
}

#app p {
  white-space: pre-line;
  margin-top: 0;
  background-color: #f5f5f5;
  width: 90%;
  font-size: 15px;
}
<!DOCTYPE html>
<html lang="ja">
<head>
    <meta charset='utf-8'>
    <title>Vue App</title>
</head>
<body>
<div id="app">
  <textarea v-model="message" placeholder="複数行入力できます"></textarea>
  <h4>Message:</h4>
    <p>{{message}}</p>
</div>
<script src="https://cdn.jsdelivr.net/npm/vue@2.5.13/dist/vue.js"></script>
<script></script>
</body>
</html>