center grid

div {
  display: inline-grid;
  grid-template-columns: repeat(3, 1fr);
  grid-gap: 3px;
  justify-items: center;
  align-items: center;
}

.center {
  display: grid;
  place-items: center;
}

<div class="container">
    <div class="row align-items-center vh-100">
        <div class="col-6 mx-auto">
            <div class="card shadow border">
                <div class="card-body d-flex flex-column align-items-center">
                    <p class="card-text">Some quick example text to build on the card title and make up the bulk of
                        the card's content.</p>
                    <a href="#" class="btn btn-primary">Go somewhere</a>
                </div>
            </div>
        </div>
    </div>
</div>