There are very few aspects of layout that are based on InDesign document pages that export meaningfully to reflowable EPUB. Most things like vertical centering and such either don't work, or don't work predictably because reflowable EPUB essentially does not have pages. More precisely, a reflowable EPUB is one page that is broken into display pages by the actions of the reader. So vertical positioning is amost entirely controlled by top and bottom spacing on each style, and it varies whether space-above at the top of a page is "collapsed" or not.
It's best to make your page layouts not depend on precise vertical spacing, or rely on ID features such as vertical justification. Most won't work out as intended. As with the name issue, apply spacing that will fit within the smallest reasonable reader page, so that larger pages will have some space at the bottom but smaller pages won't break up the content.