The victims are aged 30 and 29, news site The Korea Herald said in a Thursday report, citing Gyeonggi Nambu Provincial Police and authorities of Anseong City, where the accident happened.
The older brother came to South Korea around seven years ago before his brother followed five years later. They live in an apartment in Anseong along with the older brother's wife, a Vietnamese migrant, said the report.
It said the two men were killed after the highest floor of a nine-story shopping complex under construction collapsed onto the floor below.
The brothers were rushed to a nearby hospital, but they were declared dead shortly after arrival.
Police said they were pouring concrete and the construction formwork and the beams used to support the floor beneath failed to sustain the weight of the concrete.
The accident left four other workers injured.
Around 200,000, Vietnamese are living in South Korea, accounting for 10.4% of all foreigners in the country, according to 2020 data from the Korea Immigration Service.