Good shout. Is the one way valve thats fitted to the vac advance pipe between plenum and dizzy really required as that looks like a potential failure point.
Cant see why you would ever want to do that on a road car- disconnecting the vac' its only ever done on race cars that are flat out or fully shut throttle so part throttle ignition timing does not matter. You dont gain any BHP by disconnecting it- just loose MPG.