Vanishing points are a form of linear perspective that is used to create foreshortening or a sense of depth to an object. Here are a couple of examples as to how vanishing points work. With cylindrical shape, the seemingly parallel edges of the cylinder would meet at a point in the distance. The front of the cylinder would be a circle. The back edge of the cylinder should arc as if the back end was a circle also. So what in 3-dimensions is two same sized circles connected by two parallel lines becomes in a 2-dimensional drawing two different sized circles connected by two lines that eventually meet at a point. These are two very different things, but one represents what is while the latter represents what we see.