There are a few questions you need to ask your self when shopping for the right bathroom vanity. These questions will help you narrow down what features you'll need to look for and what type of mounting option.

What is the space you have available? Vanities come in a range of widths so it's important to determine the space first before deciding on other factors. Vanities come in single, double or corner units. They also have different mounting options like your standard or wall mounted.
How many sinks do you need? Most vanities come with one or two sinks. Be aware that more sinks mean more space.
What is the amount of storage you need? There are many options for storage with drawers, shelves, and cabinets.
How would you like to customize it? Vanities can come with a variety of different countertops, bases, hardware, sinks, and backslashes. These all have different materials and color schemes.