King Sisavang Vatthana was forced to abdicate in 1975 when the monarchy was abolished by the Pathet Lao. He was given the meaningless position of advisor to the president. In 1976 the King and Queen left the Royal Palace and moved to a private residence near by (I just happened to walk past it this morning). In 1977 they were placed under house arrest, and then moved to Houaphan for re-education. That's when the trail goes cold. If memory serves me right, Christopher Kremmer, in his book Bamboo Palace: Discovering the lost Dynasty of Laos
, presents fairly good evidence to suggest the King and Queen, and Crown Prince, died sometime between 1978 and 1980. In 1989 the Prime Minister of Laos, Kaysone Phomvihane, on a visit to Paris, said that the King died of malaria in 1984. While there is circumstantial evidence to support the theory that the King and Queen died in a forced labour camp, there does not appear to be any evidence at all to suggest that the King was still alive as late as 1984.
Evans, Grant. The politics of ritual and remembrance: Laos since 1975.