The air conditioner is low on refrigerant.
the air coming from the vent is warm
Maybe the muffler is bad.
Fog?
no it has a vent if it is coming out the vent it is over full.
Is this vent in a car or house??
Depending on the car it could be a vent for the fill hose. It could be a fuel line to the front of the car It could be a vent line for the charcoal canister (pollution control system) Or it could be a return line from the fuel injection system
Heat is coming from the engine.
well it could be because of the heat. if its cold out and you have the heat on in the car, then you will get hot and that will cause the window to fog.
check your furnace or kitchen.
A rotten egg odor may indicate a dead rodent or animal in or near the ventilation system, whether in a house or car.
If your vent dryer is clogged there will be no heat coming out. It will also take longer for things to dry.
yes it will run with a broken vent, but the check engine light may come on because of tank pressure, and you may leak a bit of gas.