Complex sentence: The "vertex centroid" comes from considering the quadrilateral as being empty but having equal masses at its vertices (centre of area) comes from considering the surface of the quadrilateral as having constant density.
Split into simpler sentences: The "vertex centroid" comes from considering the quadrilateral as being empty but having equal masses at its vertices. The usual centre, called just centroid (centre of area) comes from considering the surface of the quadrilateral as having constant density.