You can add my-auto to the parent <div class="col-md-6">to achieve this. This class sets automatic fields along the y axis, see the link above for more details.
It will look like this:
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" rel="stylesheet"/>
<div class="container">
  <div class="row">
    <div class="col-xs-6 my-auto">
      <span>Text to center</span>
    </div>
    <div class="col-xs-6">
      <h1>Hello</h1>
      <h1>Hello</h1>
      <h1>Hello</h1>
    </div>
  </div>
</div>
You may be tempted to use the vertical alignment utilities , but they will only work with inline, inline blocks, inline tables, and table cell elements.