NO one has provided more persuasive evidence that it was President John F. Kennedy who got the United States into the Vietnam war than James Reston in his recently published memoir, "Deadline."Describing his interview with Mr. Kennedy following the young president's summit with Nikita Khrushchev in Vienna, Mr. Reston has this to say: "I remember that Saturday morning very well. He (Kennedy) arrived at the US embassy (in Vienna) over an hour late, shaken and angry at having been delayed by an unexpected extra meeting with the Soviet leader. He was wearing a hat - unusual for him - and he pushed it down over his forehead, sat down on a couch beside me, and sighed. I said it must have been a roug h session. Much rougher than he had expected, he said." Kennedy then told Reston that Mr. Khrushchev had threatened him, warning that if the US did not agree to communist control over access to Berlin, the Soviet Union would proceed unilaterally to dominate the routes from Western Europe to Berlin. Kennedy said that he replied that the US would fight to maintain access to its garrison in Berlin if necessary. Kennedy then went on to tell Reston that he felt sure that Khrushchev thought that anybody who had made such a mess of the Cuban invasion had no judgment. "Khrushchev," writes Reston, "had treated Kennedy with contempt, even challenging his courage, and whatever else Kennedy may have lacked, he didn't lack courage. He felt he had to act." Soon thereafter Kennedy sent more advisers to the battlefront in Vietnam. "This, I thought," Reston continues, "was a critical mistake. Once Kennedy had over 15,000 'advisers' engaged not only in giving advice but also in giving support on the battlefield. US power and prestige were thought by many officials in Washington and in Asian capitals to be committed." And here is Reston's assessment of the "who done it" argument that still is being waged - of who it was that got the US into what became a winless war that killed many Americans and finally sapped morale on the homefront: "No doubt, as President, Johnson was more responsible for commiting the US to that struggle (he eventually had 500,000 Americans in the war), but in my view Kennedy started the slide." Defenders of Kennedy on this issue usually point to Robert Kennedy's denial that his brother had any intention of going to war in Vietnam. Reston writes: "Robert Kennedy, eager to protect his brother from blame, always denied that the President intended to increase the nation's commitment to Vietnam, and also denied that the Kennedy-Khrushchev meeting in Vienna had anything to do with it. But he didn't hear what his brother said to me in the Vienna embassy, and I did." This is not just another reporter telling us of how something important happened. This is James Reston, one of the most respected men in American journalism. As a young sports writer for the Associated Press, Reston once made the job-threatening mistake of sending out a message on Western Union to all the world that one horse had won when another horse in the close finish had actually been the winner. It was at the Grand National Steeplechase at Aintree outside Liverpool where Reston made this terrible blunder (he had never covered a horse race before) "because," he writes, "I was not directly facing the finish line, but saw it from an angle and therefore saw it wrong." "So," a man in the press box that day told him, "you have to be square with the finish" in order to get it right. "I never forgot that phrase," Reston adds. And he did keep his job - as he says, by the skin of his teeth. So it is that when Reston tells us about his long and exciting life in the world of national and global affairs - about our wars, our presidents, and our leading politicians - we know we are hearing views of someone who has had insider knowledge of what he is writing about and who always is, indeed, seeking to stand square with the finish.