It’s not possible to ALT+TAB between the desktop viewer and a game vorpX is attached to. vorpX runs either ‘inside’ a game or the desktop viewer. It can not run inside two programs at the same time.
You can play windowed games using the desktop viewer though by disabling the vorpX watcher after starting the desktop viewer. That way ALT+TAB works, but since vorpX doesn’t hook into the game directly you lose all functionality that vorpX provides when it runs directly inside a game (most importantly stereo 3D), so that’s usually not something you should do.