When I called the Phạm Văn Đồng 1958 letter to China regarding China's 12-mile territorial sea limit as a 'promissory note', I was being extremely generous on its context.
Here is the US 1972 interpretation of that declaration...Chinese members are welcomed to correct misinterpretations, if there are any...
We can see that item 4 is of interest.
What Phạm Văn Đồng did in his 1958 letter was not so much conceding authority, custodial rights, and sovereignty of those islands to China but to merely acknowledged to China that the Democratic Republic of Vietnam, aka North Viet Nam, respect this declaration in spirit and in letter by doing their best not to violate that limit. China's declaration was broad. It cover not only geomorphological entities at seas, fancy words for islands, but also the Chinese mainland, as covered in paragraph 1.
If the DRV had any intention of conceding authority, custodial rights, and sovereignty of the islands to China, Phạm Văn Đồng would have stated specifically those islands. After all, he could not mention mainland China as a concession of authority, custodial rights, and sovereignty, could he? How absurd is that notion: Phạm Văn Đồng concede authority, custodial rights, and sovereignty of mainland China => China?
Imperial Japan was very specific in her concession...
Treaty of Peace with Japan
So what prevented Phạm Văn Đồng from doing the same? Nothing. He could be as specific or as vague as he wanted. And he did the latter.
So if Phạm Văn Đồng knew that he
COULD NOT give away authority, custodial rights, and sovereignty of mainland China to...errr... China...
...That mean he knew that the DRV, despite its claim to represent all of Viet Nam, never had authority, custodial rights, and sovereignty of the islands but that the RVN, aka South Viet Nam, did...!!! And that it was the RVN who could give away authority, custodial rights, and sovereignty to the same.
What made this 1958 letter to China a 'promissory note' is that Viet Nam was in a civil war and the DRV needed all the help the PLA could provide. Phạm Văn Đồng was deliberately vague to give China the impression that Viet Nam, once unified under the banner of communism, would repay China for her assistance and if
THAT were to happened, Viet Nam would have been as specific as Imperial Japan was regarding the same islands.
Sorry (but not really sorry) to our Chinese members here, but this 1958 note is worthless.