Both; the clothes dryer vent and the laundry roof exhaust should be equipped with a damper to prevent back-drafting. Those dampers aren’t air-tight; they are usually very flimsy, but sufficient to prevent infiltration.
Since you’ve mentioned the sewer gasses odor to be more noticeable after the house hasn’t been occupied for a longer period of time, I would consider another possibility. All the traps under the plumbing fixtures are normally filled with water to prevent sewer gasses from entering the house.
If plumbing fixtures are used on regular basis those traps are being constantly refilled with fresh water. However, in vacant homes the water slowly evaporates from a trap which opens the path for sewer gasses. Maybe this is the problem you’re facing.