The filter solution is not a fix because once the carbon as absorbed all of the contaminates, they will need to be changed out. You plumber should be able to determine the number of connections. Another is if all of these vents are in the same attic area, you might want to join some of them to a single vent, but you have to be careful since so much vent is needed for certain size pipes and amount of drains. To experiment that this the problem, temporarily insert a PVC pipe in the lower vent to get it higher in the air to see if it helps. The lead boot would be installed over the vent pipe and the top of the lead boot is turned down inside the stack. With a lot of vents it is very possible that as one point when water flows or not flowing there is a direct connection from one vent to another without any traps in between. Vent pipes are normally flashed with a lead flange and sleeve (collar or boot). As air is being suck out of one vent and the higher pressure vent is providing air into the vent. Probably when you have the problem air pressure over one vent is much lower than another. When water flows down the drain it pulls air into the vent, therefore, it is rare to smell any odors. The vents near the top of the roof will have a higher pressure over it than ones near the bottom edge of the roof. I assume you have pitched roof and as the winds blows, it is the same as the air flowing over an aircraft wing.

With a lot of stacks scattered around the roof and the wind blows, the air pressures over the pipes are different.

Lead vent stack flashing