Complex sentence: Bùi Viện, the founder of Haiphong city, was a famous reformer in the XIX century, and the first (known) Vietnamese been to the United States and then the Ambassador to the United States under the Nguyễn Dynasty.
Split into simpler sentences: Bùi Viện, the founder of Haiphong city, was a famous reformer in the 19th century. He was the first Vietnamese known to have visited the United States and also served as ambassador to the United States under the Nguyễn Dynasty.