If Phạm Văn Đồng's 1958 letter to China constitute concession, then what are we to make of President Ngô Đình Diệm's 1961 incorporation of the islands into Quang Nam province? How did that North Viet Nam's 1958 letter overrode South Viet Nam's resistance to China's claim to the islands? Not only common sense but international law recognizes that only the claimant -- or 'the State' -- that has effective custodial controls of a territory have the right to do with the territory as it see fit. To exploit or to give away if it want to. Did North Viet Nam had such effective custodial controls of the islands to give it away to China? No.
In another way of looking at any moral value and legal force of this 1958 letter, when France withdrew from the region, she conceded whatever territory she controlled at that time to the best available authority figure she believed to be most capable of assuming authority, exercise proper custodial duties, and establish visible sovereign claims on the islands: South Viet Nam. This government then resisted, successfully or else is besides the point, plans and attempts by foreigners to establish their own authority upon the islands. Should we not consider that resistance to be as equally valid an attempt to deny Chinese possession of the islands as North Viet Nam's alleged concession of the same? And if that resistance is equally valid, then should they morally cancel each other out? And if we agree that they canceled each other out, then should we place any value, moral and/or legal, of the current Chinese presentation of that 1958 letter from Pham Van Dong? No, it has no moral value and no legal force. Then and today.
