Edit in JSFiddle

.container{
	display: flex;
	align-items: center;
	height: 10em;
	background: #7396a1;
    width: 300px;
    color: #fff;
    margin-left: auto;
    margin-right: auto;
}

<div class="container">
	<p>I'm aligned vertically!</p>
</div>