I pilot the plane cessna in the simulator for a long time, and I always saw this VAC indication next to the Ammeter. What is VAC for and how to read it?
In short, it is the vacuum gauge. It measures the vacuum pressure in Inches of Mercury. It runs/powers your non-electrical gyroscopic instruments. Those are your Attitude Indicator, your Directional Gyroscope, and sometimes your Turn Coordinator. These are all necessary to fly by Instrument Flight Rules.