The explosion took place at around 9:45 a.m. as plumes of black smoke billowed from the plant run by the Foosung Precision Industry Co. in Wonju City, around 130 kilometers (80 miles) to the east of Seoul, the Yonhap News Agency reported.
Two Vietnamese workers, whose names have not been revealed, have been confirmed dead and their bodies transferred to the Wonju Medical Center, the report quoted a firefighter as saying.
Two other Vietnamese workers suffered extensive burns and were rushed to a local hospital for treatment, it said.
One worker in his 20s suffered third-degree burns to his face, while the other, aged 30, had second-degree burns to his legs and a broken left leg.
An official from the hospital said the injured workers are being treated, but refused to say anything about their health status.
The explosion is being investigated by South Korean authorities.
Officials of the Foosung Precision Industry Co., which makes fluoropolymer-lined tanks, pipes and valves, declined to give any details of the accident.
South Korea is one of the largest labor export markets for Vietnam. There are over 45,398 Vietnamese migrant workers in South Korea by the end of June 2018.
The majority of them are registered under the Employment Permit System, according to official labor reports.
The Vietnamese also account for 30-40 percent of around 200,000 illegal foreign workers in South Korea.