Modal Vertical-Middle Div - Do not works

when i put modal box code in vertical-middle div, it do not works properly

<div class="vertical-middle>
 
                                                        <button class="btn btn-primary" data-toggle="modal" data-target=".bs-example-modal-lg">Large modal</button>
 
                                                        <div class="modal fade bs-example-modal-lg" style="z-index: 21;" tabindex="-1" role="dialog" aria-labelledby="myLargeModalLabel" aria-hidden="true">
                                                            <div class="modal-dialog modal-lg">
                                                                <div class="modal-body">
                                                                    <div class="modal-content">
                                                                        <div class="modal-header">
                                                                            <h4 class="modal-title" id="myModalLabel">Modal Heading</h4>
                                                                            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
                                                                        </div>
                                                                        <div class="modal-body">
                                                                            <p>4 Marine Supply is a member of Euro Marine Group and is the only company supplying the marine industry in Georgia. 4 Marine Supply is a member of Euro Marine Group and is the only company supplying the marine industry in Georgia. 4 Marine Supply is a member of Euro Marine Group and is the only company supplying the marine industry in Georgia. 4 Marine Supply is a member of Euro Marine Group and is the only company supplying the marine industry in Georgia. 4 Marine Supply is a member of Euro Marine Group and is the only company supplying the marine industry in Georgia. 4 Marine Supply is a member of Euro Marine Group and is the only company supplying the marine industry in Georgia.</p>
                                                                        </div>
                                                                    </div>
                                                                </div>
                                                            </div>
                                                        </div>
 
</div>