Two men from the central province of Quang Binh died Sunday after several mortar bombs exploded when they were hunting earthworms for fishing.
Officials said Dinh Da, 24, and Dinh Ngot, 23, might have triggered the bombs during digging.
Locals said the bombs were likely from the Vietnam War area. They added that the blast occurred on a road that served as a major route for arms and ammunition transport during the war.
Unexploded ordnance left behind from the war remains on a fifth of all Vietnamese landmass and officials have said explosions happen every day.
The explosions kill more than 1,500 people and injure 2,200 in the country every year, Deputy Minister of Defense Nguyen Chi Vinh said in October.