I'm probably missing some important concept here, so here goes:
The first virtual server (domain) that I create for a new user has its DocumentRoot on /home//public_html/. Any subsequent servers (domains) are considered "subservers" (huh? what if the user drops the main domain and wants to keep the others?) and have their respective DocumentRoots on /home//domains//public_html/. That's a redundant "domains" (public_html has a per-domain subdirectory anyway) and a redundant .
I guess I'm unclear about the subserver concept, but I found no way of establishing a second main server for an existing user.
So, any enlightening words appreciated :)