In addition to what @Joe and @ramin said, you should set up 5 domains in the following manner for the best installation experience with Virtualmin:
Create and set domain.tld and www.domain.tld to IP address of the server on which Virtualmin will be installed. These domains will become the first hosted domain that you will use under Virtualmin.
Create and set vps.domain.tld to IP address of the server on which Virtualmin will be installed. This will be the hostname that you will use for your Virtualmin server.
Create and set ns1.domain.tld and ns2.domain.tld to IP address of the server on which Virtualmin will be installed. You will set these to be the primary and secondary nameservers for your Virtualmin server in the Install Wizard.
With this simple pre-flight setup, your experience with the installation of Virtualmin and its future use will be a smooth one.
I wish this info were to be displayed in the Install Wizard. It will help many first time users prep their domain for a successful Virtualmin install and lead to a wider adoption of Virtualmin.