In the Ports tab of the Network Interface / Card Type detail that contains the port to which a Virtual sub-port will be assigned, highlight the relevant port, and click to create a new virtual sub-port.

 

There is no option to choose another card to assign the Virtual sub-port, it is automatically assigned to the selected Network Interface/card type.

 

On creation of a component from this type, a virtual port is created on the component with the same name.