Voltmeter with one-wire alternator?

Did you ground the other side of the voltmeter?

Yeah, attached the ground from the ammeter and reused the bulb too. Do I need another ground?

Can you draw a sketch of how you wired the voltmeter up? The alternator would have no bearing on the voltmeter. A voltmeter is simply measuring voltage to ground. The pink wire you connected to for +12 volts should have power when the ignition switch is in the run position. Have you verified that?

The meter has a ground connection in addition to the case being grounded for the light. One post to +12v and the other post to ground.

If you take a ground from the daisychain of grounds feeding the OEM gauges you are tapping into the circuit that is controlled by the dimmer potentiometer.

I am thinking you need a solid chassis ground instead of the ground you are using

Larry

Unlike the OEM Veglia gauges, the case of the Hall voltmeter isn't grounded, you need to attach a ground to the post marked negative and switched power to the positive post.
If you use the bulb that comes with it, you need to connect a ground to one of the bulb wires and use the stock amp meter's bulb positive on the other side so it will dim with the other gauges.
I ran a "fresh" switched 12V wire from my fuse panel so I can't answer where the best place to get switched 12v back there is.

I’m using my old bulb. I will go to a fresh ground tomorrow and see if that works. Thanks.

I'm using my old bulb holder as well (because I'm using Pantera Electronics' LED bulbs) so I soldered a ground to the case since the stock bulb holder doesn't have a ground wire (it has that little tab that friction fits against the bulb socket).