The Kobo application on the Kobo Vox generally ignores the CSS in the epub, and does its own thing. You can actually download 2 versions of most books from Kobo - one that goes directly to the Kobo, and one in real epub format for other readers.
If you want to see the epub on the Vox as you created it, use the Mantano application. There is a free version, that you can get from
http://slideme.org/application/mantano-reader-free
Keith