Could you set up two sequential user programs (registrations) or OTS's, one with VH off and the other with VH on? Then you could program two foot switches with "next program" and "previous program" to turn the VH on and off.

A clunky workaround, I know. But if the OS lacks this function, you'll have to get creative. Also see my other post about a Midi Solutions Footswitch Controller. If VH can be controlled thru MIDI that's the way to go (and not terribly hard to implement, either.)