The East Sea (South China Sea) remains a flashpoint of maritime disputes, where China's policy of "setting aside disputes, pursuing joint exploitation" has emerged as both a strategic instrument and a legal maneuver to reinforce its expansive maritime claims. This article traces the historical formation and core content of China's joint development proposition, critically examining its implications for Vietnam’s sovereignty, sovereign rights, and legitimate interests under international law, particularly the 1982 United Nations Convention on the Law of the Sea (UNCLOS). The paper argues that China's approach not only attempts to legitimize unlawful claims but also poses strategic and legal risks to Vietnam. In response, the article proposes a comprehensive set of countermeasures for Vietnam across legal, diplomatic, operational, and public communication domains to safeguard its maritime interests in the East Sea.