Attached is an instruction of how to implement a vacuum supervision to the picking process.
webwiz/148/VacSwitch.zip
Feedback of users is welcome 
Some feedback of such kind application:
We have try to use this but due to the waittime it was not possible to reach our cycle time of 100 picks/min.
In our case we use a sensor (faster than a vacuum switch but bad for wiring) combinated with an later action at the next pick.
I explain : In our application it was, by loss of product, not necessary to stop the robot but it does not matter according to the result you can react in different way.
By loss of product it was for us necessary to redo the pick/place sequence therefor we made a presence check (trap routine) like explain in the example but insteed of doing an item.Ack we made an Item.Nack and then at the next pick we check the presence result to do an item.Ack. With this solution we could reach our cycle time of 100picks/min and more.
I hope this could help somebody.
BR,
Fabrice