[Exim] Local user / virtual user

I think i've gone trough all docs/faq's and the relevant parts of the book
now. But i still can understand how to tackle, what must be, a simple
problem.

i want to have both "physical" localusers which get their mail dfelivered to
~/Maildir, so far, so good.

But i also want a stack of "virtual users" (poptoaster), which gets their
mail delivered to /var/spool/vdom/${domain}/{localpart}/Maildir (or some
such).

1. How do i set up an alias file (which is what i would reqwuire i guess?)
to define which virual users exist?
2. how do i fix the director for it?

Hints/tips/pagenumbers welcome.