How about a solenoid to pull the stop lever? Wiring for momentary power to shut off the engine will take some thinking.

You could fit a secondary vacuum source to the shut-off valve. When the remote start controller pulls +12V from a closed when energized VSV the shut-off valve will do its job. You might need a check valve in the vacuum line to the key switch and one along the VSV line.

Sixto
95 S420
87 300SDL