One way to easily vertically center a div is to set it to be absolute, set the width, height and then the top, bottom, left and right all equal to 0. Something like this:
.vertically-centered-box {
position:absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
width:100px;
height:100px;
margin:auto;
}