I would say that the most important lesson to be learned about diplomatic negotiations from this war is that diplomatic negotiations between countries that are at war or are enemies only bear any fruit if one side feels that it needs to capitulate. In other words, countries do not come to agreements because the agreements are just or because they are the right thing to do. They only agree if they are forced to do so.
We see this lesson time and again during the Vietnam War. One or both sides will refuse to negotiate, or will refuse to make a particular deal because they think that they can do better through war or other means. They only negotiate or agree when they are forced to do so.
As an example of this, North Vietnam only agreed to negotiate in late 1972 because it was being hurt by the war. US bombing campaigns and the ground war were inflicting significant losses on the North. It was also worried about the way in which the US was looking to improve relations with China and the Soviet Union. The North worried that they might become isolated if the US initiated good relations with the major communist countries. Therefore, it was willing to come to the negotiating table where previously it had not been willing.
The main lesson of this war, in diplomatic terms, then, is that power talks and countries only compromise when they feel that they must.