Seems to me there is an electic operated vacuum switch in the W123 series cars.
I think I found it in Fastland parts. It could be key operated to close off vacuum when running and open vacuum to the IP shutoff solenoid when the key (and power) is off.
I'll be using one to keep the engine running until purged in a vegoil system I've installed in my '82 240D.

Jim
San Antonio TX
'82 240D 4sp with proto vegoil system
'83 240D 4sp 86K too nice to experiment with