I dont know about the switch mechanism at the ignitions switch. but you could check the brown line that goes into the firewall to the ignition switch and then to the fuel pump. You may have a vacuum leak with that line or elsewhere as vacuum loss is not discriminating.
So course of action
1 test the brown line that goes to the switch (have it all set up) Does it hold pressure? If it does you have a leak elsewhere. To double check start the car with your vacuum tester on the brown line. start the car then apply direct vacuum to only that circuit via the brown line. if the car stops then your leak is elswhere.
3. If the brown circuit is leaking try changing all the rubber vacuum hose that you can at the hose connection points.
4. This problem of vacuum leaks sucks...you have to investigate all of the vacuum systems as they all effect eachother. you have source vacuum and that is good. Your vacuum leak could likely be climate control related. How has that system been working. What you do is check all the systems one at a time and see which one is faulty. apply source vacuum directly to the system that you are checking while disconnecting the other systems (plug only the brown line into source vacuum aka vacuum booster line.)
That should get you started
Also I am guessing that your vacuum leak is not from your ignition switch. either a disonnected or poor connection or vacuum element from the climate control.
also there is alot of links to vacuum leaks.
I know your pain