As Bulldog has suggested above, two voltmeter models in particular that have established a good track record with some VN750 forum members include the following:
Model #1050 from Signal Dynamics--
and the LED gauge unit from Kuryakyn--
Actually, both voltmeters can be seen with the last link shown above--just scroll down enough after it loads to see and read about either or both.
The advantages to these units include:
1. They have already been designed specifically for motorcycle use (i.e., tested for vibration and water-tightness) and require no real modifications to themselves
2. Since they give the necessary feedback via color
readout, they promote safety by keeping the rider more quickly informed (by just a color
check) and thus more focused on traffic and riding
3. Early warning of a charging system failure is more immediate, via a <<<flashing>>>
4. Both are less than $40, and either is easily worth the investment
5. They have been around long enough that any "bugs" in their designs have been already worked out (e.g., the previous night-brightness problem with the Kuryakyn model).
When a bike's charging system starts to fail, these units will usually give more than enough satisfactory early warning to make it (back) to civilization before the bike actually stalls out or fries a battery.
Installation questions can generally be easily answered by one of the forum members.