The victims were reportedly part of a group of workers from Duyen Hai Thermal Power Company in Tra Vinh Province, who were tasked Thursday with removing mud from the water collection tank of the company's pumping station.
While working, four of the workers fainted and were rushed to a local hospital, where they were pronounced dead.
Initial investigation suggests that the victims' cause of death was inert gas asphyxiation.
The number of workplace accidents in Vietnam has remained high despite authorities’ efforts to improve labor safety and health.
According to the Department of Work Safety, Vietnam saw 8,956 workplace accidents with 9,173 victims and 928 deaths in 2017.
In the first half of this year, there were 3,988 workplace accidents that killed 384 people.