~War in Vietnam was centuries old before Kennedy was born.
The Japanese occupied French Indochina during WWII. The US under FDR and HST aided Vietnamese freedom fighters, called the Viet Minh, a large number of whom were led by Ho Chi Minh. After the war, the French, with the help of the British and Indians, and with US blessings (double-crossing Ho and the Viet Minh) regained control of their Southeast Asian fiefdoms, Vietnam included.. The Viet Minh and other Vietnamese nationalist groups and patriotic organizations frowned on that and continued their fight for independence. Any chance the US had for friendly relations with Ho and the Viet Minh were severely damaged. Truman was at the helm then.
When the French got their butts kicked at Dien Bien Phu in 1954 and pulled out of Indochina, they warned Eisenhower not to get involved (because of the overwhelming support of the Viet Minh and because a war against them could not be one) and especially to avoid Ngo Dinh Diem, calling him an incompetent madman. The Geneva Accords signed that year to resolve the political situation in Indochina required that elections be held in 1956 to reunify Vietnam. It was clear that Ho Chi Minh would win the elections, in the North as well as the South. He would have headed a coalition government composed of communists and Vietnamese nationalists, both of which groups felt betrayed by the US and neither of which were going to knuckle under to Washington and US interference in the region.
Ignoring not only the French but also centuries of Southeast Asian history, Ike sent in the first US advisers (not counting the ones sent in to help Ho Chi Minh and the Viet Minh during war). Compounding the error and further ignoring the French and common sense, the US backed Diem. With US help, Diem stole the election. Diem said he was going to win by more than 90% of the vote. The US advised him his margin of victory should only be in the 60 to 70% range to lend some credibility to the process. Diem said the election was a test of his power. In a "fair" US sponsored election, Diem received votes from more than 150% of the registered voters in several districts and won by 98.2%.
During his first year in office, Diem had imprisoned mere than 100,000 political opponents. When that didn’t work, he started killing them wholesale. The Viet Mihn was the benefactor of Diem’s ruthlessness. The more corrupt he became, the bloodier his hands, the more support the Viet Minh got. They were NOT a North Vietnamese group – rather, they were Vietnamese nationalists and patriots. Other groups, such as the National Front for the Liberation of South Vietnam (NLF) formed, also in support of the reunification of Vietnam and against Diem. Diem was Catholic. More than 70% of the Vietnamese were Buddhist. Diem, as had the French before him, repressed the Buddhists. This drove even more of the populace into the “enemy” camp. Meanwhile, Ike ordered the CIA and US military to protect Diem and his government. When the time for the mandated elections came, Diem refused to allow them. Ho was going to win by an even greater margin than before. The US wasn't about to tolerated that (having just deposed two democratically elected anti-American socialists in Guatemala and Iran - CIA Operations PB/Success, PB/History and TP/Ajax), so they backed Diem again and helped prevent the elections. The fighting heated up and more and more Vietnamese joined the ranks of the Vietminh, NLF and similar groups. Convinced that the democratic process was not going to work as long as Diem was in charge and the US was helping and protecting him, they resorted to force of arms. This all happened on Ike's watch.
Such was the situation Kennedy inherited. JFK increased US troop strength from several hundred to 16,000. JFK ordered the CIA and US military to continue to protect Diem and his brothers, who Diem had installed in top government positions from which they went on a killing spree the likes of which would have made Stalin proud. They also cornered the vast opium market. Finally, Kennedy had had enough. He was considering pulling out. That was no secret and the Vietnamese were aware of it. But first, Diem had to be dealt with. JFK sanctioned and the CIA financed and otherwise assisted in a coup that resulted in the assassination of Diem on November 2, 1963, just 20 days before Kennedy bought it in Dallas. If anything, the Vietnamese would have applauded Kennedy for his role in Diem’s murder and they surely did not oppose the newly hatching idea of the US stopping its interference in the region. (Those sentiments would have been universal, in the North and South alike.) After a string of additional coups, the equally bloodthirsty and corrupt Nguyen Van Thieu took office, with US blessings. Meanwhile, the Vietnamese people knew what was going on and the role the US played in it. But killing Kennedy wasn’t going to solve their problems. They needed the elections they had been promised, but denied by US puppets, and, through them, by the US, to do that.
[And the US was there to promote democracy, freedom and human rights?]
Bobby Kennedy always believed the CIA had a hand in his brother’s death. Bobby believed it was because JFK refused to invade Cuba, though, and not because of Vietnam. Bobby also suspected the military top brass might have been involved because JFK believed he was the commander in chief and he didn’t let folks like Curtis LeMay play with their nukes or take on the Soviets. There was so much animosity between the Kennedy and the CIA, FBI and Joint Chiefs that his stand on Vietnam was a minor bone of contention.
Then, of course, one has to consider the way JFK and RFK double-crossed Jimmy Hoffa as well as all the Kennedy mob buddies who helped JFK steal first the primaries then the general election - particularly Carlos Marcello, Sam Traficanti and Momo Giancana. Then there are the Cubans ex-patriots in the US who JFK lied to for years, not to mention Castro, who was a little tired of US interference on his island and the numerous attempted assassinations of him that the CIA bungled, not to mention the Bay of Pigs, which the CIA, not JFK, botched, although JFK may have deliberately sabotaged the invasion [although the CIA did forget to tell him that the Soviets and Castro were aware of not only the invasion plan but the landing sites and date and time as well, and the CIA forgot to tell him that their own analysts predicted failure without US air and/or naval support, which Kennedy absolutely refused - before the fact - to provide: that was one of the few promises Jack kept]. The Soviets are a non-issue. Khrushchev actually liked him and the Politburo knew they could back him down (after all, they built the Berlin wall and got JFK to pull the US missiles out of Turkey and had previously forced the US to recognize the Pathet Lao (also not exclusively a communist group) as a legitimate force in Laos). LBJ never liked the man, either, and he wanted to escalate 'Nam, not back away from it. He also wanted the top job, and he had a lot of friends at the Pentagon, although not as many at CIA.
If you are suggesting Oswald didn't act alone or needed a reason, there a plenty of viable candidates who could have helped him and far better reasons than JFK's Vietnam policy. But it was likely that Kennedy was going to lose the '64 election so killing him was a rather drastic and unnecessary way to deal with him.