The only option that I see here, that is even remotely simple would be a power close solenoid, which is I think what you are describing you have. So that with power on it closes, no vacuum to shut off, but if the main switch was flipped it would open and apply vacuum to the shut off

Another option I suppose would be some sort of flap that clamps off the air supply to the engine. If you could find an actuator that pulled against a spring it could work. Power on, the actuator opens the air flap, power off the spring slams it shut.

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold