"In 1961, Kennedy agreed that America should finance an increase in the size of the South Vietnamese Army from 150,000 to 170,000. He also agreed that an extra 1000 US military advisors should be sent to South Vietnam to help train the South Vietnamese Army. Both of these decisions were not made public as they broke the agreements made at the 1954 Geneva Agreement."
"by the end of 1962 there were 12,000 ... advisors in South Vietnam. As well as sending more advisors to South Vietnam, Kennedy also sent 300 helicopters with US pilots. They were told to avoid military combat at all costs but this became all but impossible to fulfil."

http://www.historylearningsite.co.uk/vietnam-war/john-f-kennedy-and-vietnam/