DNS things, as usually

[kilburn]

Ispcp 1.0.7 already creates accounts for "vuXXXX.master.domain.tld" for any domain that you add to the system. Thus, you can simply add the users as if they had real domains.

The only issue with this setup is that you must make sure that you set their number of mails limit to -1 (disabled), or otherwise the server will not be able to send e-mails to the corresponding real domain.