The engine has a number of sensors that it can fall back on if one is duff or reading an unexpected value. For example the HEGO sensor will allow trimming of the air fuel mixture even if the detected air flow is wrong so as to ensure the engine keeps running.
For a given engine speed the car knows how much air should be coming in; if due to a vacuum leak, that mass is wrong, you will get detected low air mass, quite correctly. It could be a MAF sensor, it could be a thousand things, but I would be checking for leaks first.
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