Korea, that had been occupied by the Japanese from 1910 until 1945, was partitioned into two areas, north and south of the 38th parallel. A pro-western regime had been set up in southern Korea and a Stalinist regime was installed in north Korea. Such settlement had not been satisfactory for Koreans of any political affiliation, and tensions remained critical until an all-out war broke out in 1950.
Before being able to talk about “success” and “failure”, a definition of these terms is necessary, especially in such a complex situation. In 1945, after the Empire of Japan’s defeat, Americans had refused to meet with the legitimate government of the People's Republic of Korea, a center-left provisional government which had been established by Koreans after the end of Japanese occupation. They also had had their strongman, Syngman Rhee, a controversial figure of independence activism, flown in from Tokyo, in MacArthur's own personal airplane, to assume a leading position in the formation of Korea’s government. In 1948, constitutional assembly elections were held and the newly elected government, appointed him as the first Korean president. It is important to point out that his first election was not the result of a popular, national vote-casting but an appointment by the government, under the pressure of Americans. Rhee had studied and lived in the United States for a long time and was considered as the most pro-American choice by Washington. He was a fiercely anti-communist and authoritarian figure and he outlawed left-wing politicians, labor leaders and other political dissenters. His security forces tortured and massacred tens of thousands of activists under the silent watch of American military authorities, particularly in the Island of Cheju. In later elections he would have members of the Korean National Assembly arrested and the constitution changed in order to be re-elected. In 1960, after elections which were considered to be rigged by most of the population, Rhee was ousted by a popular uprising and was again flown out of the country towards Hawaii by Americans, where he lived until his death in 1965. Was this a success or a failure? The Southern part of the country was kept in the American capitalistic sphere of influence (and has slowly evolved to a more democratic regime) but the northern part of the country is still a Stalinist, hereditary dictatorship. The best qualification for this would be a successful failure.
Germany was an actual success. After the defeat of the Nazi regime the country was divided into Allied and Soviet occupation zones, with the city of Berlin being divided along those ideological lines as well. Because Americans did not deny West Germans what they had denied to so many other countries all over the world (free elections, generous social protection network, labor rights, etc.) West Germany remained firmly in the Western political and economic sphere. East Germany became a satellite state with a puppet regime that obeyed the Soviets. It citizens were constantly watched and controlled and had only one idea in mind: to flee to the West, notably to West Germany. At the end of the Cold War the country was reunified and is now the most powerful economy of Europe and an example of democracy and living standards for the world (though the system is increasingly starting to fail due to the rampant ultraorthodox capitalistic policies of its governments).
Vietnam was an absolute disaster. The Japanese occupiers left after the end of WWII and the French resumed colonization as if the war had never taken place. The Communist guerrilla fighters had all been educated in both France and the Soviet Union or sometimes Communist China. Americans thought the French would able to suppress them and keep the colony. They were eventually forced out militarily by Communist guerrillas and the US failed to help the French understand that their best option was to grant the country its independence before they got kicked out. Had the US done that, the country would not be a Marxist dictatorship but a Scandinavian-style, social-democratic, capitalistic society. After that, the US tried to reproduce the Korean example and by doing that it lost all credibility as a moral beacon, and suffered its most devastating political defeat during the 20th century. All the lives and money losses were staggering, especially for the Vietnamese. Even worse, the Vietnam War spread to Laos and Cambodia and resulted in totalitarian, Stalinist regimes. In Cambodia, the situation evolved into genocide where three million people were exterminated by the Stalinist/Maoist guerrillas.