The vermicompost bin provides a home for your worms and the entire vermicompost process. A good bin will provide the correct environment with plenty of drainage and air flow. Bad drainage will allow water to collect or pool in the bin, this will lead to rotting and odors.
Without air the worms and other good organisms die, this also leads to rotting and odors. Maintaining good airflow through the bin keeps the vermicompost process running smoothly. Bins should never be sealed or wrapped in plastic.
I have seen many bins designs, but for me the stacked tray model is the best. Each tray has a mesh bottom that allows worms to travel freely, along with full drainage and good air flow.
With this design, compostables are added to the top of the bin while finished vermicompost is harvested from the bottom tray. The empty bottom tray is then added back to the top and each tray move downward as they are being composted. This creates a system that can run indefinitely.