Final answer:
The concept that extends Vitruvius's idea of symmetry to achieve proportional harmony is called Proportional Balance, which aims to establish a harmoniously balanced composition through the relative sizes and arrangement of elements in a design.
Step-by-step explanation:
The concept that builds off of Vitruvius's idea of symmetry to create proportional harmony is Proportional Balance. This approach considers the relative sizes and distribution of elements within a design to achieve a harmonious and balanced composition. It can be seen throughout history in the architecture and art of various cultures, including the use of the golden ratio by the ancient Greeks. Proportional harmony seeks to find an aesthetic balance within a work that feels natural and pleasing to the eye. Examples include the symmetry of bilateral designs and the asymmetrical yet balanced distribution of visual elements known as contrapposto in sculpture.