Two 13-year-old boys drowned at an excavation site in the central-coast resort town of Phan Thiet on Friday.
According to rescuers at Doi Duong beach, Nguyen Minh Thanh and Pham Hoang Khang were found dead in a three-meter-deep hole dug for the construction of a dyke.
The hole, located at the seashore, is usually buried under sea when the tide is high, but the contractor, Thai Binh Company headquartered in the northern city of Hai Phong, didn't erect any signboards, local people said.
After the incident the company sent a bulldozer to fill the excavation.
Rescuers said a few days ago they had rescued five local students who also fell into the hole.