Treason is defined in Section 3 of Article III of the United States Constitution:
Treason against the United States, shall consist only in levying War
against them, or in adhering to their Enemies, giving them Aid and
Comfort. No Person shall be convicted of Treason unless on the
Testimony of two Witnesses to the same overt Act, or on Confession in
The Congress shall have Power to declare the Punishment of Treason,
but no Attainder of Treason shall work Corruption of Blood, or
Forfeiture except during the Life of the Person attainted.
Nixon did not levy war on the United States. Nixon did not adhere to the enemies of the United States (a term that means those against whom a declaration of war has been made). And, Nixon did not give aid and comfort to the enemies of the United States in the sense described. He met with the South Vietnamese Ambassador, our ally, not the North Vietnamese ambassador.
How to make peace in order to end a war is a matter in the sole discretion of the President.