Nguyen Van Bay is just a kind of nickname that means "he's the seventh son". Especially back in those days of large families it isn't massively surprising that

Nguyen Van Bay is just a kind of nickname that means "he's the seventh son". Especially back in those days of large families it isn't massively surprising that there'd be another seventh son in the airforce. Vietnamese Wikipedia says he actually went by the nickname Bay A (literally 7-A) in the air force, presumably because of the other guy also nicknamed Bay. The Nguyen family name is obviously super common. Van is a fairly common middle name. Especially in the south of Vietnam it was somewhat common, especially back then to just call people by their birth order. "Aunt Eight", "Uncle Two", "Sister Three", etc. His actual name was Nguyen Van Hoa.