Cadin,
A few possible problems with your plan:

1. When the key is in "run" or "start", the vacuum switch vents the shut-off valve tube. Your VSV would need to be a similar SPDT vacuum switch to disconnect the path to the key switch.

2. Your VSV should not actuate while cranking the engine, before oil pressure has come up.

An easy indicator would be to tap into an existing warning lamp, such as the "brake fluid low" or "clutch pads worn" sensors. I recall those simply short to ground to actuate the bulb, so the "low oil" switch could be another short to ground. That is how most 1-wire switches, like in my 60's Chryslers work. Of course, a future owner may never figure out the lamp.