A vector image is made up of points, lines and curves. A bitmap is made up of tiny microscopic pixels, only visible under magnification.
Bitmap format is useful when attempting to retain soft details. Yet, scaling can create quality problems. When a bitmap is saved smaller, pixels are basically thrown away. When a bitmap is saved larger, pixels are added somewhat randomly (or interpolated by the computer); therefore, the image can appear less detailed or down-right nasty.
Vector images can be scaled from a tiny logo to a colossal billboard at no loss of quality. Vector format is easily manipulated similar to the stretchable quality of a rubber band. Yet, if a photographic quality is necessary, at least at this point in time, vector is not the ideal format.
Bitmap is my format choice for the haiku illustration project, since I would like to retain soft details contained in my marks, especially for the lines, "petals ruminate kindly," and "dimly breezily." The delicate nature of petals and also a soft breeze would be difficult to capture with a harsh, crisp vector format.