I want to deliver mail for virtual users (users that dont have an
entry in the /etc/passwd file) to separate mailboxes in
/var/spool/mail/vhosts/ directory.

The hostname and the domainname are the same.
My main.cf for postfix currently looks like this (the relevant bits):
#########
virtual_transport = virtual
virtual_mailbox_base = /var/spool/mail/vhosts/
virtual_mailbox_maps = hash:/etc/postfix/vmailbox
virtual_mailbox_domains = $mydomain, $myhostname
virtual_minimum_uid = 100
virtual_uid_maps = static:505
virtual_gid_maps = static:505

local_recipient_maps =
local_transport = local

mydestination = $myhostname, localhost.$mydomain, $mydomain
#########

The file /etc/postfix/vmailbox looks entirely like this

vmtest@domainname.com     vmtest

###########################################

What am I doing wrong? According to the manuals, this should deliver
mail to the mailbox /var/spool/mail/vhosts/vmtest, but is currently
rejecting mail saying unknown user (I am catching the mail in another
pure user mailbox).

Note: I have already received the following answer and it did not
prove to be sufficient.
"Take a look at: http://www.gentoo.org/doc/en/virt-mail-howto.xml
that is the how to I have used to setup my Virtual Mailbox system.
This should work out just fine for you if you follow it point by
point."

Regards,

J

When asking questions about postfix, two things are nearly always a must.

1) The output from ``postconf -n''
2) Logging information, most probably from /var/log/maillog or /var/log/mail.log

Regarding 2, find the queue ID of the appropiate message and do a 

    $ grep <QUEUEID> /path/to/mail.log

e.g.

    $ grep 3A9DC2D /var/log/maillog

Phil.