Your are correct that idle is the highest point of vacuum in the intake manifold in all normal cars. . BMW Vanos is slightly different in that the throttle plate is controlled by the computer and is not used for rpm control and defaults to fully open when the engine is running. That's why there is a supplementary vacuum pump.

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2005 X5 4.4i SterlingGrau, Sport Package, DSP
Build 3/12/2005