Vacuum is like that bad jalepeno- it always comes back. Think of it as a controlled vacuum leak. When driving, the throttle blades are somewhat open as air and fuel are rushing into your combustion chambers as the piston moves down the cylinder (conversely moving air through the PCV system as well). The engine will eat the fuel and air mixture to get to an equillibrium rpm if you hold it in one spot, assuming load doesn't change. Close throttle a bit, you reduce the air/fuel coming in, vacuum goes up, and rpm starts to come down (starving for fuel). Mash the throttle to the floor and you have an instant "hole" basically ruining your vacuum. It's your accelerator pump on the carb, or fuel supply device design based on throttle position that is controlling your fuel and air. If you're system is working right, the hole gets covered up with fuel leading to an instantaneous blast of rpm. If it doesn't, you get the bog, and the car falls on its face.
The 9" at idle tells me something isn't right. A 510 lift cam shouldn't do that unless you're running a 2 cylinder. Timing? Vacuum leak? Bad rings? Something seems odd with that. Obviously I have no idea about the engine, but even my cammed 455 makes around 14 at idle (and shifts fine with the vacuum modulator).
