You can use the following HTML to align items / div vertically center on the screen:
<div class="container d-flex flex-column min-vh-100 justify-content-center align-items-center">
<div class="row">
<div class="col">
<h2 class="font-weight-light">Hello Bootstrap</h2>
<p>
This is a how you make a div vertically centered on a screen.
</p>
</div>
</div>
</div>