How can you improve the venting on a bathroom sink

Most bathroom drain slowdowns and blockages start at the popup assembly, not at the trap or in the other drain lines. Garbage disposal odor is the result of food particles getting stuck to the inside of the disposal chamber and decomposing. This most often occurs when you don't run the water long enough while using your disposal, and the rotting food matter can build up over time, creating an ever-worsening odor. To flush out the disposal and eliminate the smell, put the stopper in the drain and fill the sink with about a gallon of soapy water.

The venting is probably and usually fine, but check it for a blockage. However, a fairly recent discovery to most odor problems was finally linked back, by apparent morons (only took them 100-years), to be coming from sink overflow hole. Try bottle brushing both your overflow drain and its pipe and your sink drain down to the trap with bleach. That, should remedy the problem for a few years, if not longer.