What about this?
‘’”When The Wall was built, it was thought the first American military casualties were occurred 1959. After The Wall was erected, two families brought forth proof that two American military advisors were killed by artillery fire in Vietnam in 1957. Their names were added to panel 1E but it was decided to not correct the carved date, which would deface The Wall.”’’
http://www.virtualwall.org/aboutwal.htm
The point of the article is that the VC began specifically targeting Americans July 8, 1959. The casualties before that were the result of other actions. At Bien Hoa they began to bring the war to us explicitly.