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Sorry, I meant to say "A vacuum leak on a MAP-controlled system...". I have found that air leaks that are close to the intake valve actually do affect the mixture. Checked with a WBO2 meter, and because the car idled like crapola.
This is in a VW-based car, and a relatively-ancient one at that. (The finest 1960s-vintage analog electronic fuel injection!) It has fairly long intake runner pipes from the manifold to the intake ports (on the order of 14" or so) and our working theory was that the "false air" didn't really have time to propagate back up to the manifold to be measured by the MAP sensor.
As I said, though, simply dumping air into the manifold (e.g., disconnected vacuum line or a deliberate leak of some kind like "evap") will not affect the mixture. So you are correct in the case we were actually discussing--just not necessarily in all other cases.
-soD
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