It should be pretty steady. You might have a problem and you might not?
If each cylinder is pulling in the "exact same" amount of air the needle will stay perfectly still because the vacuum does not change as each piston takes its turn sucking. If for some reason one cylinder or more is pulling in less air it causes the needle to momentarily drop because the amount of air coming through the intake lowered during that intake stroke
If the needle is steady but low the problem is something that is effecting all of the cylinders the same. Timing, low compression.....
If you see a steady, systematic drop. Something is "mechanically" wrong with one cylinder.. Burned valve, hole in a piston, broken valve spring, worn cam lobe...… When you have a big systematic drop the next test is to do a compression test.
If one cylinder does not fire because of an ignition or fuel problem the crankshaft rotation will slow for just a fraction of a second. Mechanically that cylinder is good and was able to create a good strong vacuum because another cylinder was firing and pushing our non-firing piston through its intake stroke, just like what normally happens. When its turn comes to fire and it does not, that slows the crankshaft rotation speed for just a fraction of a second that it would have an effect on how much air was drawn through the intake causing the needle to fluctuate just a little. As Ron said above, if the air fuel ratio is not correct they might not all be igniting creating the same pressure.
Something is not 100% the same in one cylinder as the others or all the cylinders are not consistently creating the same pressure on the pistons. The second part could be an air fuel mixture problem like Ron mentioned.
Adjust the air fuel mixture using a propane bottle. Add a little propane and if the rpm goes up the engine is too lean. Add propane and if the idle goes down the engine is too rich. Add "a little" propane and the rpm does not change the engine is at its happy spot. Gives the A/F mixtures screws another 1/8 turn rich for all around driving conditions.
This chart says a 1 to 2-inch constant fluctuation can be caused by spark plug misfire or a slight valve leak. A spark plug might not fire the cylinder because there was not enough fuel molecules between the center electrode and ground for the spark to ignite. Spark plug "not firing" does not always mean that it did not make a spark. Make sure your air fuel ratio is correct
Shut off the engine pull the plug wires and just set them back on the plugs so they make contact but can be easily removed. Start the engine and with the best insulated plyers money can buy lift one wire off at a time and see what effect it has on the rpm and the vacuum reading. Compare the rpm drop and the vacuum drop of all the cylinders. If they are all the same you probably don't have a problem.
Warning, never remove a spark plug wire on a running engine with electronic control modules or computers. The voltage is always looking for a way to get to ground. If it cannot jump the gap when you pull the wire it will find another way to get to ground. It will backtrack through the plug and it might just find a path to ground through the electronic module. With electronic ignitions they make a special dummy spark plug that you clamp to a ground and connect the plug wire to it before checking the cylinder. Its not the same as trying this test with a grounded loose spark plug. You need to buy the tool. They are made internally different.
Here is website describing how this special spark plug works. Its only for testing HEI ignitions.
https://easyautodiagnostics.com/misc-index/hei-spark-tester
