Forefront IT Services Ltd

You will need to edit the slimbox.css for VirtueMart:
Slimbox is in /components/com_virtuemart/js/slimbox,

#lbOverlay {
position: absolute;
left: 0;
background-color: #000;
cursor: pointer;
z-index:1000; <--Add this
}

#lbCenter, #lbBottomContainer {
position:absolute;
left:50%;
overflow:hidden;
background-color:#FFFFFF;
z-index:1000; <---Add this
}