On June 15, 1956, Premier Pham Van Dong reportedly said to China: "From the historical point of view, these islands are Chinese territory"
(Beijing Review March 30, 1979, p.20 -- Also in Far East Economic Review March 16, 1979, p. 11).
In 9/1958, when China, in its declaration extending the breadth of Chinese territorial waters to 12 nautical miles, specified that the decision
applied to all Chinese territories, including the Paracels and the Spratlies, Hanoi again went on record to recognize China'sovereignty over the 2 archipelagoes.
PVD stated in a note to Chinese leader Zhou Enlai on 14/9/1958: "The Government of the Democratic Republic of VN recognizes and supports the declaration of the Government of the People's Republic of China on its decision concerning China territorial sea made on 4/9/1958
(see Beijing Review 19/6/1958, p.21 -- Beijing Review 25/8/1979, p.25 -- The existence of such a statement anf its contents were acknowledged in VN in BBC/FE, no. 6189, 9/8/1979, p. 1).
In 1975 Vietnam occupied a number of islands in the Spratly archipelago and subsequently pressed territorial claims to the entire South China Sea. As Foreign Minister Nguyen Manh Cam has admitted:
"Our leaders' previous declaration on the Hoang Sa (Paracel) and Truong Sa (Spratly) archipelagoes was made in the following context: At that time, under the 1954 Geneva agreement on Indochina,
the territories from the 17th parallel southward including the two archipelagoes were under the control of the South Vietnam administration. Moreover, Vietnam then had to concentrate all its force
on the highest goal of resisting the US aggressive war to defend national independence. It had to gain support of friends all over the world. Meanwhile, Sino-Vietnamese relations were very close and
the two countries trusted each other. China was according to Vietnam a very great support and valuable assistance. In that context and stemming from the above-said urgent requirement, our leaders' declaration
[supporting China's claims to sovereignty over the Paracel and Spratly islands] was necessary because it directly served the fight for the defence of national independence and the freedom of the motherland.
More specifically, it aimed at meeting the then immediate need to prevent the US imperialists from using these islands to attack us. It has nothing to do with the historical and legal foundations of Vietnam's
sovereignty over the,Truong Sa and Hoang Sa archipelagoes"
(remarks to a press conference in Hanoi on 2 December 1992 carried by Vietnam News Agency, 3 December 1992).
Later, Pham Van Dong denied his past wrongdoing in an issue of Far Eastern Economic Review, March 16, 1979. Basically, he said the reason he did was because it was "wartime".
Here's excerpt from this article on p. 11 : "According to Li (Chinese Vice-Premier Li Xiannian), China was ready to share the gulf's water "half and half" with the Vietnamese,
but at the negotiating table, Hanoi drew the line of Vietnamese control close to Hainan island. Li also said that in 1956 (or 1958?), Vietnamese Premier Pham Van Dong supported
a Chinese statement about sovereignty over the Spratly and Paracel islands, but since late 1975, Vietnam has been in control of part of the Spratly group - the Paracels being under
Chinese control. In 1977, Dong reportedly said of his 1956 stance: "That was the war period and I had to say that".