The only function of a vent is to prevent the trap on a fixture from being siphoned dry. The dry trap is how sewer gas would get into the house. If the toilet bowl has water in it, then it is not dry. For a sink or shower, you can look straight down and see water...that is water IN the trap. A tub you can't see....you can test with your nose, then pour a quart of water in slowly and see if the smell goes away.