You don't really need a virtualhost for access to the webmail if you don't want it. You could use a folder under the DocumentRoot of an existing host or virtualhost.
The webmail's root folder could be in /var/www/html/webmail people would access it like this..
www.yourdomain.com/webmail
virtual hosting is just a cleaner way to seperate things, and could also allow certain restrictions on the virtualhost that is not on other virtualhosts.