The example code for navs using vertical tabs (https://getbootstrap.com/docs/4.0/components/navs/#javascript-behavior) leaves out that it internally creates a row and puts the links in one column and the tab content in another. You have to inspect the code of the bootstrap documentation to see how it it actually works on that page.
It should really look similar to:
<div class="bd-example bd-example-tabs">
<div class="row">
<div class="col-3">
<div class="nav flex-column nav-pills" id="v-pills-tab" role="tablist" aria-orientation="vertical">
<a class="nav-link active" id="v-pills-home-tab" data-toggle="pill" href="#v-pills-home" role="tab" aria-controls="v-pills-home" aria-selected="true">Home</a>
<a class="nav-link" id="v-pills-profile-tab" data-toggle="pill" href="#v-pills-profile" role="tab" aria-controls="v-pills-profile" aria-selected="false">Profile</a>
<a class="nav-link" id="v-pills-messages-tab" data-toggle="pill" href="#v-pills-messages" role="tab" aria-controls="v-pills-messages" aria-selected="false">Messages</a>
<a class="nav-link" id="v-pills-settings-tab" data-toggle="pill" href="#v-pills-settings" role="tab" aria-controls="v-pills-settings" aria-selected="false">Settings</a>
</div>
</div>
<div class="col-9">
<div class="tab-content" id="v-pills-tabContent">
<div class="tab-pane fade show active" id="v-pills-home" role="tabpanel" aria-labelledby="v-pills-home-tab">...</div>
<div class="tab-pane fade" id="v-pills-profile" role="tabpanel" aria-labelledby="v-pills-profile-tab">...</div>
<div class="tab-pane fade" id="v-pills-messages" role="tabpanel" aria-labelledby="v-pills-messages-tab">...</div>
<div class="tab-pane fade" id="v-pills-settings" role="tabpanel" aria-labelledby="v-pills-settings-tab">...</div>
</div>
</div>
</div>
</div>
The example code for navs using vertical tabs (https://getbootstrap.com/docs/4.0/components/navs/#javascript-behavior) leaves out that it internally creates a row and puts the links in one column and the tab content in another. You have to inspect the code of the bootstrap documentation to see how it it actually works on that page.
It should really look similar to: