Its easy to check the vacuum shut-off valve by the position of the lever on top of the Injection Pump. Normally if the valve is faulty it will usually fail due to vacuum leaks and the car won't shut-off from the key switch. You then have to stop it using the "STOP" lever.
I guess its possible that this lever is stuck in the stop (down) position, but a quick look at its position should tell you right away.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown