In Debian virtual hosts are enabled by default, so after you have configured your dns drop a file named as you like (e.g. site.example.org) in /etc/apache2/sites-available with this content
# This tell apache to enable this vhost for all ports
# These two are the only mandatory params for a vhost
ServerName your.dns.name # dns name for this vhost
DocumentRoot /your/document/root # document root for this vhost
# Optional, allow override in .htaccess files
# optional, log accesses and errors to a different file
CustomLog /var/log/apache2/your-name-access.log combined
Then as root run the command
a2ensite site.example.org (the file name) and reload apache with
/etc/init.d/apache2 reload as suggested by previous command. That's it.
If something goes wrong review your config with
apache2ctl -t for syntax check and with
apache2ctl -s for virtual host configuration.