Every now and then we run into a radiator that still has a vacuum vent on it, or a customer wants to replace a radiator vent and they are confused since they cannot find a replacement vacuum vent and that is what they had before.
The bottom line is that unless you are still operating a coal boiler and firing it with coal, you don't need a vacuum vent. They were for coal boilers. They are not for gas or oil boilers.
Whats the difference? Good question. A valid question since your desire is to replace an air vent, not the boiler.
A gas or oil boiler will fire intermittently. Its either on, or its off. When its off there is absolutely ZERO production of steam. If the system were allowed to drop into a vacuum it would be a wickedly deep vacuum that perhaps could even do damage. The water will disappear from the sight glass completely, and the boiler gauge will pull backwards and usually break. Its just not a good thing to induce.
Now think back to the original coal boiler. It was fired up in October and ran perhaps till April or May. Lets think about your charcoal bar-b-que. You fire it up and the coals give off their heat. Then what? Do the coals just turn off? No, they slowly start to die down. The coal boiler did this too. It was stoked, lit, made a good fire, made lots of steam, then the coals started to die down, and the pressure would also start to drop. As the steam production drops off due to the coals dieing down, air would normally re-enter the system. But the vacuum vents wouldn't allow this to happen. Instead as the coals start to die down, the system starts to drop into a vacuum, but steam is still being produced because the boiling point of the water is now much lower than 212 degrees. Because of the vacuum! The water may boil as low as 180 degrees or even lower. See the vacuum vents were there to induce a vacuum in the piping as the coals started to die down. This way the weaker coals could still make the required amount of steam. The key here is that the system wasn't in a severe vacuum. Just deep enough to do the job. Plus steam is still being produced and the expansion from water to steam is a counter force to the vacuum. Eventually if nobody stoked the boiler, the coals would slowly die out. No system is perfectly air tight so air would leak in and eventually break up the vacuum.
So if you run across a vacuum vent on one of your radiators just replace it with a standard air vent. Chances are that the vacuum air vent was also a real slow venter and today we want to get the air out faster because its removal is again required after each and every time the boiler shuts off. Its that intermittent firing of the gas boiler again. See how intermittent firing changes the whole complexion of a system?