Это делает работу: http://jsfiddle.net/sRmLV/1140/
Вам необходимо использовать новый DIV и пользовательский CSS, изменить класс 'modal-dialog-центрированный' на 'vertical-align-center'
HTML
<button class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">Launch demo modal</button>
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="vertical-alignment-helper">
<div class="modal-dialog vertical-align-center">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal"><span aria-hidden="true">×</span><span class="sr-only">Close</span></button>
<h4 class="modal-title" id="myModalLabel">Modal title</h4>
</div>
<div class="modal-body">...</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
<button type="button" class="btn btn-primary">Save changes</button>
</div>
</div>
</div>
</div>
</div>
CSS
.vertical-alignment-helper {
display:table;
height: 100%;
width: 100%;
pointer-events:none;}
.vertical-align-center {
/* To center vertically */
display: table-cell;
vertical-align: middle;
pointer-events:none;}
.modal-content {
/* Bootstrap sets the size of the modal in the modal-dialog class, we need to inherit it */
width:inherit;
max-width:inherit; /* For Bootstrap 4 - to avoid the modal window stretching
full width */
height:inherit;
/* To center horizontally */
margin: 0 auto;
pointer-events:all;}