Put a Z wave relay on the common output line going to the valves of an existing irrigation controller. That way you can remotely shut it off. Further, you can put extra irrigation cycles which are normally disabled by the z wave controller like irrigation every second day or second time in the afternoon.
What Relay are you referring to?
Fibaro dual relay/Evolve FM-20 OR make one yourself using appliance module (I still have load of Radioshack) or wall switch powering 110V external relay. Use any pair of contacts as disabling switch.
but don’t modify the vera to turn it off, better yet use the normally on contact.