The sailor, MD Milon, 25, was on his way from China to India on the cargo ship Izumo that is registered in Panama, Vietnam News Agency reported.
When the ship was 103 nautical miles northeast of Nha Trang in Vietnam's Khanh Hoa Province, he started suffering from "a terrible chest pain" on Tuesday, said Nguyen Xuan Binh, director of the maritime search and rescue coordination center in region 4, which oversees the area off Vietnam's south-central coast.
The crew on Izumo contacted the center for help, receiving immediate assistance.
Around 1:30 a.m. on Wednesday, the SAR 27-01 rescue vessel ship reached Izumo and rushed to take MD Milon to a hospital in Nha Trang for emergency aid.
The Bangladesh sailor might have suffered from a heart attack, according to the report.
He is now in stable condition.