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Check the trap in the sink drain. The "plug" of water in the u-shaped pipe should keep sewer vapors from coming up the sink. If the trap is not deep enough or leaks, this could be the problem. I'm not a plumber, but I'm pretty sure the "vent" to the roof for the drain pipes is to let the water flow when you flush or run the sink, not to control odors? (like the vent on a gas can)
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