Sounds like you attached the vacuum switched backwards or otherwise incorrectly.

Does the key switch behave the same way when the vacuum switch isn't attached?

When you say the vacuum switch holds or doesn't hold vacuum, how do you have the vacuum gauge hooked up? The supply side should always hold a vacuum; at the switch in run or start (99% sure), at the actuator in off or accessory (100% sure). The actuator side will never hold a vacuum in the way you'd test; vented at the swtich in run or start (100% sure), vented at the non-running engine vacuum pump in off or accessory (50% sure, not sure how check valves come into play).

Sixto
87 300D


Sixto
87 300D