Final answer:
The first postmodern building designed by Robert Venturi is the Vanna Venturi House, considered a pioneering work in the postmodern architecture movement.
Step-by-step explanation:
The name of the first postmodern building designed by Robert Venturi is the Vanna Venturi House. Despite Robert Venturi's insistence on not attempting to create a new movement, the Vanna Venturi House has been widely regarded as the pioneer of postmodern architecture, a movement that emerged as a critical reaction to the strict rules and perceived lack of variety in modern architecture. The house exhibits typical postmodern characteristics such as complexity, contradiction, and the use of non-orthogonal angles, which challenge the established norms of building design.