I don’t think I understand what you’re saying.
A zone (when you register a domain, a zone is created for that domain) has NS records, and they delegate authority to DNS servers. That kinda feels like a domain pointing to name servers, to me.
Your registrar is responsible for those NS records…they’re the glue records in your domain zone. They can point to your name servers managed by Virtualmin, or they can point to the registrars DNS servers, or some other DNS servers. When you do a whois on a domain (or dig with appropriate options), it’ll show those NS records:
$ whois virtualmin.com|grep 'Name Server'
Name Server: ns1.virtualmin.com
Name Server: ns2.virtualmin.com
Those two name servers can then delegate (point to) other name servers to be responsible for subdomain names under that domain name. Or all the names and subdomains can be served by those DNS servers without delegating anything.
@Brook, you seem to have Virtualmin Pro, so I’m pretty sure you can configure Virtualmin to host DNS on the domain.com server instead of having multiple DNS servers. (Virtualmin Pro recently got the ability to somewhat easily act as a name server for other Virtualmin servers. This has always kind been possible, by running a local BIND instance on the other Virtualmin systems and having the “main” server setup as a secondary for all of those others. This used to only be possible with Cloudmin Services, which is part of Cloudmin Pro/Cloudmin Connect.)
Without Pro, you can still just create A records on the domain.com server using Manage Records. You don’t need to delegate authority for the subdomain, if you’re not doing anything complicated with it that would make having a separate zone useful. I wouldn’t go to all the trouble to manually setup all the delegation rules unless you really have a complicated DNS situation and you need to allow another user to manage the subdomain zone. Zones are about delegation; if you’re managing everything yourself, there’s no reason to delegate.
Don’t do anything complicated if you don’t need to. It’s all just names.