Virtual Nameservers.

I have a question about the virtual nameservers.
I read that they are basically treated like any other nameserver is, but it's added on top of the current nameserver.
If I used the nameservers that my hosting provider provides me, instead of registering my own, could I add 2 own virtual nameservers (without IPs) and then use these to resolve the subdomains of the domains (since my hosting provider's DNS isn't updated when I update the DNS settings on Direct Admin for my domain).
If I got that right, it'd mean that I don't register the virtual nameservers anywhere, since there are no IPs assigned for them, but this would also mean that I need to use my hosting provider's nameservers for my domain's DNS settings where my domain is regged at (since the virtual nameservers are only valid for Direct Admin I assume).

So basically my question is:
If I used my host's nameservers as my domain's 1# and 2# nameservers where my domain is regged at and then created 2 virtual nameservers on Direct Admin and assigned them for my domain in DNS settings of DA, and if I added a subdomain such as 'jeff.mydomain.com.' would it work and resolve the jeff.mydomain.com fine?

No that is not how DNS works. It is possible to set it up that way but it would take work on both ends and they probably won't do it for you.

DNS works by looking for the nameserver associated with a domain. So you have abc.com. In your DNS records you have ns1.def.com and ns2.def.com. You type in subdomain.abc.com the computer will only know to request information from ns1.def.com and ns2.def.com. It will not know to go looking somewhere else so it won't find the domain.

You COULD, it could cause problems add your ip's for your server on the DNS record. The primary's won't have the info but your DNS would. This is really a bad hack and should't be done it could cause problems and delays while tring to resolve the DNS.

You also have the solution that many smart ISP do. You hide the master DNS server. It is NEVER accessable for resolving from the public internet. You could have a spin on that, your DA server could have all the DNS done on it. Ask your ISP to be the secondary DNS server for your zone and you won't need to change the DNS record.