To enhance the design, consider adding an additional layer of panels.
<div class="container-fluid main">
<div class="row">
<div class="col-md-2">
<ul class="nav nav-pills nav-stacked">
<li role="presentation" class="active"><a data-toggle="collapse" data-target="#overview" aria-expanded="true" aria-controls="overview" data-parent="#accordion">Overview</a></li>
<li role="presentation"><a data-toggle="collapse" data-target="#view1" aria-expanded="false" aria-controls="view1" data-parent="#accordion">View 1</a></li>
<li role="presentation"><a data-toggle="collapse" data-target="#view2" aria-expanded="false" aria-controls="view2" data-parent="#accordion">View 2</a></li>
</ul>
</div>
<div class="col-md-10">
<div class="panel-group" id="accordion" role="tablist" aria-multiselectable="true">
<div class="panel">
<div class="panel panel-default collapse in" id="overview">
<div class="panel-heading">
<h3 class="panel-title">Overview</h3>
</div>
<div class="panel-body">
Panel content
</div>
</div>
</div>
<div class="panel">
<div class="panel panel-default collapse" id="view1">
<div class="panel-heading">
<h3 class="panel-title">View 1</h3>
</div>
<div class="panel-body">
Panel content
</div>
</div>
</div>
<div class="panel">
<div class="panel panel-default collapse" id="view2">
<div class="panel-heading">
<h3 class="panel-title">View 2</h3>
</div>
<div class="panel-body">
Panel content
</div>
</div>
</div>
</div>
</div>
</div>
</div>
If necessary, tweak the inner content classes to remove any excess padding/margin. Keep in mind that Bootstrap automatically conceals children panels under a data-parent unless they correspond with the data-target triggered by clicking.