The vacuum is created by the engine running -- it's a gasoline powered engine, and they are "throttled" == that is, the power developed is controlled by a valve in the intake manifold (the throttle valve) that regulates the amount of air going into the engine. Naturally, the valves and pistons pull considerably against the nearly closed flap at idle, generating vacuum.

When you have a leak around the air flow meter, insufficient fuel is added, so it runs too "lean" and can stall, also won't produce good power. When you open the throttle suddenly under these conditions, it will take longer than normal for the air flow meter flap to move, resulting in throttle lag. Will work fine at higher speeds were most of the air is going past the open throttle valve.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!