You are welcome to use a third party DNS provider (many people use their domain name registrar). If you do that, you’d need to create your DNS records there, and have them point to your IP address.
Alternatively, you could setup your own server to act as a nameserver for your domains. That requires some more up front work, but makes creating Virtual Servers easier, as Virtualmin can then handle creating all your DNS records for you.
There is some info on setting up your server as a nameserver here in the section “How do I setup nameservers for my server”:
As a courtesy to the Virtualmin community, I offer some FREE assistance with getting setup. If you’d like to get familiar with this truly powerful software and learn how to optimize for your needs, drop me a line on Skype anytime.