The only purpose of the VTADD command is to add additional virtual terminals (or printers) to TPX while TPX is running.
You cannot modify existing VTAMLST members while TPX is running, so the only option if you do not wish to bring down TPX to add the new terminals is to add them via a new member in VTAMLST. VTADD will result in the new member being recognized by TPX.
NOTE: If you must use FREE=CLOSE on your VTAMLST DD, this prevents TPX from being able to add this new member while TPX is running, thus rendering the VTADD command unavailable in your environment. You will need to cycle TPX in order to add new virtual terminals or printers.