Technically that’s a subdomain, but I want to be able to move this Wordpress site around and have no real dependancies on the main site- so I should maybe create a whole new virtual server?
Is that correct or is there other considerations please?
You should create a whole new virtual server, unless you want one management user and home dir for both.
Sub-servers are about ownership and management. They have nothing to do with the name of it. A subdomain is just a name to Virtualmin. It does not care what you name your virtual servers (there are some bits where Virtualmin can manage subdomain names as part of the parent zone, in a sub-server situation, but that’s mostly irrelevant from a user perspective and not necessarily a good thing).