Both Vialle AMS but slightly different

I happen to own two SAABs, a blue 2.1i and a black 2.0i which are both bi-fuelling on regular gasoline and LPG.

While inspecting both installations I discovered that the blue 900 has a vacuum valve which is not connected and the small vacuum hose is capped.

When i plug the vacuum hose to the valve the engine shuts down instantaneously. On the black 2.0i the engine runs very smoothly on LPG.