Yes, your vanity does need vanity lights. Vanity lights help with functionality and ambiance at your vanity. By having vanity lights, you are increasing the lighting in the area while also creating a better experience for you at the vanity.
While the size of the vanity light is up to you, you should choose a vanity that is at least 75% of the width of the vanity. More specifically, choose a light 4 inches shorter than the size of the vanity. The lights can also be wider, but it just depends on your preference and the amount of illumination the fixture provides. For the best results, center the vanity light above your vanity mirror.