You can do that by checking the ‘Allow manual activation’ checkbox in the config app. Afterwards you can enable/disable the virtual display from the vorpX tray icon’s right click menu.
*VERY* IMPORTANT CAVEAT:
Having an invisible display attached as an additioal monitor introduces various pitfalls, which is why things are handled like they are handled per default. Only do that if you have a very good reason and know what you are doing.