<VirtualHost IPADDRESS:80>
ServerName existing1.domain.com
ServerAlias www.existing1.domain.com
DocumentRoot /home/user/public_html/existing1
</VirtualHost>
<VirtualHost IPADDRESS:80>
ServerName existing2.domain.com
ServerAlias www.existing2.domain.com
DocumentRoot /home/user/public_html/existing2
</VirtualHost>
....
<VirtualHost IPADDRESS:80>
ServerName *.domain.com
DocumentRoot /home/user/public_html/non-existing/
</VirtualHost>