The youngest World War I veterans would have been born around 1900. By the beginning of America’s involvement in the Vietnam War, they would have been in their late 50s to early 60s.
If you date US involvement from 1955, certainly among senior staff. General Samuel Tankersley Williams (1897 - 1984) commanded the MAAG from 1955 to 1960. Williams fought in WW I:
Unlikely. My uncle, who enlisted in the Army shortly before Pearl Harbor, served a tour in Vietnam in the early part of the war, and was considered ancient.
The first American with the official title of military commander in Vietnam was Paul D. Harkins, born in 1904, who graduated from West Pont in 1929. His successors, Westmoreland and Abrams, were born a decade later. Both Chiefs of Naval Operationsduring that era were born in 1906. If the senior leaders hadn’t been around for WWI, I seriously doubt anyone junior to them would have been.
Upon review, I see yabob researched deeper than I did.