Hi,
i was intrigued by the question so i looked at what i could find...
Looking at the database in phpmyadmin there are a couple of relevant tables it seems:
psa->accounts (don't know yet)
psa->clients (list of clients and their logins w/o mention of subscriptions)
psa->db_users (empty on my fresh install, looking at column names should be individually created db users from w/i subscriptions)
psa->domains (list of all created domains relating column "cl_id" to client id from psa->clients)
psa->filesharing_users (don't know yet)
psa->ftpusers (ftp logins where "sys_user_id" column refers to psa->sys_users->"id")
psa->mail (list of email accounts (no aliases) where "dom_id" relates to psa->domains->"id")
psa->mail_aliases (list of mail aliases where "mn_id" relates to psa->mail->"id")
psa->pd_users (list of usersnames from password protected web directories where "pd_id" relates to psa->protected_dirs->"id")
psa->smb_users (list of all clients & additional system users that where created? not entirely sure)
psa->sys_users (list of userlogins managing subscriptions (ftp login/ webadmins(?, maybe rather in psa->web_users which is still empty at my site) etc where "home" shows login dir)
A simple
grep vhosts /etc/passwd
on the shell also shows logins from psa->sys_users are created as local system users with (in my case) /bin/false as shell.
In addition, there are the commands under (in my case) /opt/psa/bin and /opt/psa/admin/bin/ where
/opt/psa/bin/
client
customer
reseller
subscription
user
webuser
seem relevant but don't have a -list option afaict. Some can check existance of accounts though and "-h" switch shows available options.
Also
/opt/psa/admin/bin/
usermng -h
shows a deprecated "--get-users-list" switch that will result in some formatted /etc/passwd output including vhost accounts.
There's more i'm sure and the above info may be incomplete/ inaccurate (beware, i just looked at this for the first time).
Getting a list should be simpler than diving into the db, but hey... Maybe i'm missing something.
hth
aeg