Have something always vertically centered of vh

How do I get an element to always be centered of viewport height? I’ve played around with the css toolkit plugin but with no luck. I’ve managed to get the viewport height, but as we can’t set heights or borders width dynamically, this isn’t of much help. I’ve even tried with floating groups set to vertically float to both bottom and top, but I’ve never got that to work.

Any suggestions on how to accomplish this, are much appreciated. Thanks in advance.