During the Cold War, the four countries Vietnam, Cambodia, Laos and Burma formed a buffer zone between the ASEAN countries and China. China and ASEAN joined forces with the USA in opposing Soviet-backed Vietnamese expansionism. With the drastic reduction of Soviet aid to Vietnam and Laos in the second half of the 1980s, Vietnam found itself in an isolated, vulnerable position, and in 1988 its navy suffered badly from a fateful clash with Chinese forces in the Spratlys. Since then, despite a number of incidents in various parts of the South China Sea, Sino-Vietnamese relations have improved all the time. Diplomatic and party-party relations were normalised in 1991. A land border treaty was signed in 1999 and ratified in 2000, and a treaty on the delimitation of the Gulf of Tonkin was signed in 2000. The inclusion of Vietnam, Laos, Burma (Myanmar) and Cambodia as member states in ASEAN during 1995-99 did not harm Vietnam’s relationship with China. Instead the Indochinese countries seemed on their way to becoming a land bridge between China and ASEAN, in a peaceful, post-communist version of the ‘domino theory’. The paper discusses the implications of Sino-Vietnamese détente and cooperation for the disputes in the South China Sea. Will Vietnam contribute to maintaining a united front within ASEAN to oppose Chinese assertiveness? Could Vietnam and China resolve their maritime disputes bilaterally? Or could China and Vietnam jointly pave the way for a regional solution to the disputes over maritime zones and sovereignty to islands?