>> I would like to center my popups vertically on the "screen".
I don't think you can.
It's been a while since I last looked at this - so my memory may be faulty, but as I recall popup dialogs have a fixed width, but variable height. Since the height is not known, it's not possible to "center" it, at least not as it opens.
It might be possible to centre it after it opens using javascript, but as far as I know that's not built-in.
cheers
Bruce