What, specifically, does the vac gauge tell you that can impact engine health and/or mileage - that you can do something about?
Where are you measuring vacuum?
I'm desmogged and I'm using one of the nipples on the three way fitting on the manifold just in front of the carb. The other two are plugged Best if you come directly off the manifold--who knows what sorts of weird crap the carb ports are doing.
A vacuum gauge can tell you if (obviously) if you have a vacuum leak, your ignition timing is off, your valves are mal-adjusted (the more expensive gauges will recommend a good shrink), if your head gasket is blown, you oil is diluted or your rings are shot. See the chart
If you drive it so the the vacuum stays as high as possible you'll get better fuel mileage. One example of how it helps--there's meandering road that goes over some gentle hills on the way to my house. I used to chug down it in fourth gear. When I do that and watch the vac gauge I'm pulling about 4" of vacuum. Same rode, same speed in 3rd gear I'm pulling about 13" vacuum. Since I've had the gauge I've burned less than half a tank and have 145 miles on the clock, I'm thinking maybe 18 mpg and that on a rig with 297xxx miles on it. AND the valves need adjusted.
At idle my rig pulls 14-15", rock steady. I'm at about 7000' elevation which accounts for the lower number I think, that and the mileage.