Why does a vacuum leak plus propane speed the engine up??

What is a vacuum leak?

It's an unnatural path for air to be sucked directly into the intake manifold, downstream of the throttle plate.

So who cares?

Well, now the engine is getting air that the fuel ECU doesn't know about (the fuel ECU wants all intake air to come in way upstream of the throttle plate, through the Air Mass Meter; that way, the AMM can tell the fuel ECU exactly how much air is coming into the engine, and the fuel ECU can adjust the fuel/air mixture correctly).  So now there's extra air with no extra gasoline added to it; the mixture goes lean - too much air - and the engine doesn't run right.  If it's a small leak, the engine will speed up, because it's getting more air and it doesn't mind being a little bit lean.  But if it's a bigger leak, it will be starved for fuel and it will run rough.  If it's a big enough leak, the engine will run so rough that it will die.

So what about the propane?

Hey, propane is fuel, isn't it?  Mix propane and air and the mixture will burn, won't it?  Add some fuel to your illegal air source and the engine will get happy in a hurry!  YOW, lots of air and now some fuel to go with it, hot damn!  So the engine speeds up.  And there's your leak!  Once you fix it, all the intake air will come into the engine through the proper entrance, the AMM can tell the fuel ECU exactly how much air the engine wants and the fuel ECU can turn the fuel injectors on for precisely the right amount of time to deliver the proper amount of fuel.  Isn't that simple?  Yeah, Wroite! (as our Aussi friends say).