The vent you are referring to is called an overflow. The primary purpose of the over flow hole is exactly that; to have a secondary drain higher up on the sink so that if the drain is clogged at the sink, the water can bypass and prevent a flood.

The secondary benefit of this hole is that it does act as a vent to allow water to flow.

However, generally speaking, the water should drain fine in a vessel sink without an overflow. If you experience a drainage issue in a vessel sink there may be a secondary issue with the plumbing.