Until the demise of the Soviet Union, all foreign policy decisions were viewed through the prism of the cold war. As a Pole, Brzeinski would not miss an opportunity to stick it to the Soviet Union. The cold war was a world war, but the possibility of a nuclear holocaust prevented a direct confrontation between the major powers. Because of these limitations, we confronted each other using proxy wars.
Until Afghanistan, the Soviet Union only supplied other countries to fight their wars of liberation. They backed North Korea in their invasion of South Korea, but, except for some pilots, they had no direct combat role there. While they received aid from the Soviet Union and China, the Vietnamese fought their own battles.
The United States was not looking for a fight in the Korean War, but that war became a model for Vietnam, in that we tried to keep South Vietnam on our side, and leave North Vietnam Communist. We did not invade the North because China had felt threatened when we pushed the North Koreans back to the Yalu river. It was a try at a limited war.
Vietnam and Afghanistan are very different countries, very different people and very different wars. But both the United States and the Soviet Union had missionary foreign policies that drew them into these conflicts. They didn't change!