To point a second domain at an existing site/doc root do we use sub server or is there a way of just aliasing the 2nd domain in the original virtual host using Virtualmin?
Not sure if this is what you mean, but I think you mean the option:
Select your domain in the dropdownlist
Create Virtual Server
On top of the page select the 3th option (alias of…):
Top-level server | Sub-server | Alias of yourdomain.com