Domain Alias / Sub Server?

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:

  1. Select your domain in the dropdownlist

  2. Create Virtual Server

On top of the page select the 3th option (alias of…):

Top-level server | Sub-server | Alias of