michael -- between 20-22 vac at idle is ideal. i guess higher could be because of upgraded exhaust mani or free exhaust, but im not sure and ive never heard of it being a good thing. with AC on, mine goes to ~17 vac, but i never drive with AC on.
it should def. read 0 when turned off. You should be able to send it to the company to get re-dialed so check into that. Or there may be a kink in the line (runs thru firewall to the intake mani)
1st lesson in thermodynamics: for a fluid to move there must be a difference in pressure in a given volumetric system. The system is from your intake, to your cylinders. Gas is pushed out of the cylinders by the piston after combustion, which is then lifted back up creating a vacuum to suck air back into the cylinder for combustion. There must be a vacuum (or boost for that matter) for the air to move into the cylinders, and this is what your gauge is reading, measured from the intake mani just before it enters the combustion chamber.