If the horizon level is aligned with the viewer's eye, how can a vanishing point lie below or above the horizon line?
Are vanishing points defined relative to objects?
The vanishing point will be on the eye level along with the horizon line for lines that are parallel to the ground plane. If an object has lines that are not parallel to that plane they will converge to a point off the horizon.
Or if the ground in the distance is for example a hill or mountain or some other incline then the horizontal lines for a building nearby will converge below the horizon because the horizon is higher than your eye level.