The yacht, carrying around a dozen people on board including former Lao president Choummaly Sayasone (pic) and his family and friends, capsized at around 10.30am local time (0330 GMT) on Sunday when there was a sudden storm in the capital area with strong wind, thunder and fairly heavy rain.
It is learnt that Sayasone's body has not been found yet.
Nam Ngum Lake, some 60 km north of Vientiane, is a weekend destination for Vientiane citizens.
Witnesses told Xinhua via phone that a search and rescue operation has been carried out.
According to rescue staff and sources close to the Choummaly family, Choummaly was found alive and rushed to the hospital, his youngest son and two daughters were also rescued, while four bodies have also been recovered and five more people were still missing, till 4pm local time (0900 GMT).
Choummaly, born on March 6, 1936, was the General Secretary of the Lao People's Revolutionary Party (LPRP) and President of Laos from 2006 to 2016.
He was born in Attapeu of southern Laos and joined the LPRP central committee politburo in 1991 and served as the defense minister from 1991 to 2001 - Xinhua