When you create a virtualhost, you can set several FQDN domain name to reach your web content and You have a check box to create the dns zone. No problem with that all is right.
When the virtualhost has been created, you can still modify the FQDN, but you have no more the option to create the dns zone…furthermore, if you modify a FQDN, it is not removed from the /etc/hosts and the new one is not added of course.
The DNS record creation is optional and independent from vhost names because the name can be registered in an external DNS db.
This is true especially in the public DNS. From the LAN side, a little UI helper is available on creation.
We can remove it completely or support more complex use cases. I think it is available on creation only because other use cases fall in the complex category!