The fuel metering depends on the MAF (Mass Air Flow Sensor - the mechanical flap in this case) being able to measure the mass of air entering the engine. In fact, all such systems depend on being able to meter the mass of air entering the engine.

So, all air must pass by that flap if the system is to "know" the mass of air entering the engine. If any air gets into the engine by not passing the flap, it is not metered by the system and the amount of fuel entering the engine is unlikely to be appropriate to prevailing conditions.

Vacuum leaks can allow un-metered air to enter the engine, so any attempt to set fuel delivery is thwarted from the outset.

Does that make sense?

I found that certain driving conditions led me to vacuum problems. High speed driving was perfect. Idling, particularly when hot, was the most testing time for my vacuum-leaky system.

RayH
PS: I use the term MAF in this context because that is what it does. Some say that the term MAF should only be applied to the hot-wire/hot-film devices in later systems.