why do they specifically need separate sub-domains?
the URLs for an organization are:
vcloudurl.domain.com/cloud/<Org ID>
Most companies I have seen use random alpha numeric values for the Org Name (there is another spot for a friendly name). So you would get something like this:
vcloudurl.domain.com/cloud/10294424
theoretically, if you use a wildcard certificate you might be able to get away with that ... except that the public addresses part of the vCD installation could be problematic (I would advise against it).