I have been using squirrelmail for a long time, although I did not need the virtual user plugin yet.
what I do is the following: I use a mail.* virtual domain to send anyone going to mail.hisdomain.com hosted on my server to go to the same login page. so both mail.domain1 and mail.domain2 point to the same squirrelmail installation, but sm notices which domain the user logged into from and the mail is then sent from that domain.
so I had no need for the virtual user plugin, what I use is the alias login plugin, so users can pick aliases inside their personal options and pick a nick, which is then stored inside a file like: web3_admin:jack and then that user can login with jack.
there are a lot more good plugins, just to name a few: change_passwd plugin => users can change their passwd from inside sm, quota display => they see their disk quota inside sm, and many more...
could you make a short howto for configuring the virtual user plugin? I would like its functionality too, but it was too complicated for me so I am content with the current sm configuration....