At around 10 a.m. on Sunday, a group of eight women tourists from Ho Chi Minh City rented the vehicle in the Bau Trang tourist area to explore the sand dunes.
After taking photos by the lake the tourists and their driver, a man named Vong Thanh Cuong, 33, got back into the vehicle to continue their journey when the vehicle suddenly rolled backward into the lake.
The driver and several of the passengers were rescued by people nearby.
But Le Thi Thu Thuy, 56, was trapped inside the vehicle.
By the time rescue personnel arrived and took her out she was dead.
Authorities in Binh Thuan Province, home to Phan Thiet, are investigating the cause of the accident.
Bau Trang, situated in Bac Binh District, 60 km north of Phan Thiet City, is famous for its two large lakes, sand dunes and a forest.
Dubbed a "mini Sahara Desert," it attracts thousands of visitors every year.
In 2023, the foot of the sand dunes was severely eroded.
Tourism authorities repeatedly warn off-road tour operators not to take visitors to potentially dangerous areas.