[DSG2003Mach1]

1) if youre going to unhook the car's battery you dont have to unhook the gauges or anything

the whole thing is not to put the key in the ignition and turn it any direction that will put power to the add on while the gauge is unhooked from the OBD2 port. Yes, you can just pull the add a circuit before unplugging the gauge from the OBD2 port and youll be fine.

2) the vacuum source you used is fine. You either have something wired up funky or didnt program the gauge correctly (which is easy to do, it took me a few times to figure out WTF I was doing). The gauge has 2 analog inputs, you have to make sure you are setting up the correct one and that you put the slope and intercept in correctly. With it reading 13.xxx I say again it sounds like you're displaying battery voltage but Im not sure which mistake would be more likely to cause that