Is it normal to smell sewer gas from a plumbing vent?
It is normal for sewage gas to escape from drain vents. That's why they usually vent through the roof, that and unclogging. The vents tie directly into the pipes that lead to the sewers. There are plumbing traps that can cut down on the amount of gas that escapes but some will always escape. The vent cap should help but just make sure they don't seal the vent.