I looked through the manual. No listing for the foot switch to control VH on/off, but you can assign the Vocal Harmony volume to an expression pedal. That may be what DNJ is hearing, since the pedal he is using is not an expression pedal, but a momentary pedal.
Again, I would love that to happen if it were me.
Still I see no reason to remove the vh on/off from the foot switch menu. I'll bet it's a boo-boo.