I haven't been around for quite a while, because I did the impossible (almost) and empowered my humble Fairmont wagon with a full set of 1984 Mustang gauges, including the Trip meter, and then added a voltmeter and vacuum gauge. Now I can see what the engine is doing and catch fuel usage changes by the tankful.
Here's some of the things that a vacuum gauge will tell you:
Vacuum Gauge troubleshooting
If you're a Ford fan - click on the combo box for some tech info:
efv8 - Tech Tips
What I like about that graphic (right-click and save picture as...) is that it prints out into a small enough page for the owners manual. Handy on the road.
This one shows one dynamically through 15 different conditions:
How to Use and Interpret a Vacuum Gauge
Another downloadable chart for GMers:
Reading your vacuum gauge
A decent bit of tech detail about the Vacuum gauge:
Using a vacuum gauge
My car doesn't have all the bells and whistles for a Scanguage or other Hi-tech monitoring. In fact, because it was built in Mexico, it didn't even have Electronic ignition. After upgrading to a freshly dealer-rebuilt 3.3L I6 and transmission, it finally matured into the E-age.
After seeing all that a Vacuum gauge tells me, I wouldn't own a car without one! It's the early OBDII, as far as I'm concerned.
That goes for the voltmeter too. Up here, in the cold Canadian Prairies, your life depends on the charging system. The Ammeter just tells me how much the car is using. The Voltmeter tells me the system is charging, good, bad or not enough.