Installed new coil, now the tach acts funny

That tach is perfectly safe on coil neg. The only types of ignition you don't connect the tach to the coil are MSD or other CD ignitions Most all CD ignitions are aftermarket.

Yes I was referring to VR. That should not affect the tach. My thoughts were that with it plugged in, some problem was generating RFI/ EMI (think radio static) and causing electrical noise, triggering the tach

So far as the voltage checks yes, ground the meter. The alternator case should be grounded, but you can ground to the engine