In many ways, diplomacy between the defeated powers and the Allies in World War II was very limited, since the Allies demanded the complete and unconditional surrender of the Axis Powers. It took the death of Hitler in 1945 and the occupation of virtually all of Germany, including Berlin, to bring about German surrender. A peace treaty which stipulated repatriation of conquered territory and the surrender of all German troops in occupied territories was signed by Alfred Jodl in May of 1945. The German government had little choice. Prior to that, some of Hitler's subordinates had sent diplomatic feelers to the Allies, whose replies usually mandated the removal of Adolf Hitler and the unconditional surrender of all German forces as a condition.
The case of Japan is slightly more complicated, as a faction of Japanese officers and civilian officials favored surrender in the summer of 1945. But the Allies, who had total air superiority over Japan, were determined to accept nothing less than unconditional surrender. As this was believed to include the resignation of the Emperor, many Japanese hard-liners resolved to fight on, and only the devastation of Hiroshima and Nagasaki brought about Japanese surrender.
The most important diplomatic negotiations of World War II were not between Axis and Allied representatives, but between the Allied leaders themselves, and the disagreements between them over the shape of the postwar world, particularly in Europe, helped to lay the groundwork for the Cold War.