The dugongs, weighed between 250 and 500 kilogrammes, were captured and killed by local fishermen in Ha Tien town and Phu Quoc island in the southern province of Kien Giang, the Tuoi Tre newspaper said.
However, the WorldWide Fund for Nature's Indochina programme in Hanoi said they had only been informed of the death of one dugong in Phu Quoc, by local authorities.
The population of dugongs, a whale-like mammal found in shallow tropical waters from East Africa to Australia, has declined dramatically in the past 25 years in Vietnam, despite being listed in the government's conservation "red book".