Virtual email domains are a supplement to the normal email capabilities of your mail server. Normal email domains are still managed properly and the email is stored in /var/spool/mail.
So if your machine is currently accepting email for domain foo.com
and you wish to receive, separately, email for foo1.com and
foo2.com,
you just have to define those two domains as virtual email domains. The
configuration for foo.com is unaffected.