In a small bathroom, less is always more. If you include a vanity, you will probably need to choose a very small unit. Depending on your needs, you can choose the style (cabinet or drawers or a combination) that is most suited to your needs. Recessed medicine cabinets (set into the wall, with an unframed mirror) are ideal in small bathrooms.
Some people like the idea of dividing even a small vanity with cabinet on one side and drawers on the other. Larger items can be stored in the cabinet and small items can be organized in the drawers. If you have small children, you can also place locks on the drawers used to store potentially dangerous items and cleaning substances.