Sorry, but in the diplomatic note from Pham Van Dong, North Vietnam gave nothing away 1958. In the diplomatic note the northvietnamese Government only confirmed the chinese sovereignty over the Paracel- and Spratly-Islands. Since North Vietnam won the Civil War, every diplomatic note they gave and every treaty they signed are legally binding. The case would be different, if South Vietnam won the Vietnamwar.
Now to the International Court of Justice. The question is, whether the ICJ will maketheir decision independently or not. 9 out of 15 judges are definetly not pro China, so it will be like a vabanque game, if China agrees to abide by the Court's adjudication. So China has nothing to loose, if it doesn't accept the ICJ as arbitral tribunal, but China can loose everything, if it does.