Slow draining vessel sinks disappoint many people after remodeling a bathroom and installing a vessel sink. People assume many causes such as clogs and venting problems are causing the slow draining often spending money needlessly trying to correct the problem. The slow draining makes keeping the sink clean much more difficult with you not being able to rinse the sink clean after use.
The vessel sink faces two problems, the sinks do not have an overflow built into the sink and they are usually installed with a grid drain. In a sense venting causes the slow draining, but your plumbing can be vented correctly and still have the problem. Venting in plumbing is installed on the drain side of the trap and its function is to protect the trap seal from positive and negative pressures that can be generated in the drainage system. These pressures could cause the water in the trap to get sucked out of the trap and allow sewer gases to enter your home. In addition to the odors these sewer gases can cause diseases and were traced to causing an outbreak of SARS in china, in 2003.
The venting problem in a vessel sink is caused by air entrapped between the grid strainer and the water in the trap. This entrapped air blocks the flow of water down the drain slowing the flow of water leaving the sink. Often if you look down into a grid strainer drain you can see the air bubble moving around under the holes of the grid strainer, with the air trapped or, held down by the water on top of and coming through the holes in the grid strainer. The problem is this entrapped air is on the wrong side of the trap for a vent to have any effect allowing it to escape. In a sink with an overflow built into the sink, the air trapped under the sink drain can escape through the overflow allowing the sink to drain. The vessel sink without an overflow is doomed by design.
The best remedy is to avoid using a grid strainer type drain, instead opting for a lift and turn pop-up style drain. While you still may get air entrapped in the drain the problem will be greatly reduced and you probably will not end up with large amounts of water in the sink slowly draining. If you are going to keep the grid strainer drain you may be able to get better drainage by carefully drilling out the holes in the grid as large as possible without destroying the grid strainer. The larger holes may allow some of the air to escape faster before being blocked by the water on top and will allow more water to flow through the larger holes allowing better drainage from the sink. These are really the only two options that will work.
I have seen people go through great lengths attempting to solve this problem including adding Air Admittance Valves (AAV’s) also known as cheater vents on the drainpipes under the sink. This is not a solution because AAV’s are one-way valves, only allowing air to flow in, and not out. Don’t waste your time on this thinking it will solve your problem. Using a grid strainer on a sink drain that does not have an overflow is just a bad design. Some of the worst ideas in plumbing survive by being fashionable.