You could probably cram a VCD into the DVD_ROM section of a DVD. It would require some work on the part of the user to point their viewer to the right spot on the disc though and would of course not be playable as a VCD in settops.
sure there are double sided discs with CD on one side and DVD on the other. They are called Dual Discs and all of the major music labels have been pushing them as the next big thing (also called DVD+ and released on a version of Blair Witch 2 several years back with the movie on one side and the soundtrack on the other). There are, however, no writable versions of these discs...strictly replicated versions.