<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css" integrity="sha384-HSMxcRTRxnN+Bdg0JdbxYKrThecOKuH5zCYotlSAcp1+c8xmyTe9GYg1l9a69psu" crossorigin="anonymous">
<style>
body {
background: transparent;
}
.vertical-align {
display: flex;
align-items: center;
justify-content: center;
flex-direction: row;
}
.vertical-align > div:first-child {
background-color: lightblue;
}
.vertical-align > div:last-child {
background-color: lightgreen;
}
.big-box {
height: 10em;
}
.small-box {
height: 3em;
}
</style>
</head>
<body>
<div class="container">
<div class="row vertical-align">
<div class="col-xs-6 col-md-8">
<div class="big-box">Big</div>
</div>
<div class="col-xs-6 col-md-4">