Storm Haitang is expected to hit central Ha Tinh and Thua Thien-Hue provinces early September 27 before weakening to a low pressure system, according to the National Hydro-meteorological Forecast Centre.
However, another, much stronger storm, Nesat, is brewing east of the Philippines and is likely to hit the East Sea at night the same day.
Offshore vessels have been banned from putting out to sea in central coastal areas from Nghe An to Quang Nam.
Nesat is forecast to move west-northwest at a speed of 20km per hour, bringing wind gusts of up to 183km per hour.
Head of the National Committee for Flood and Storm Control Cao Duc Phat told coastal cities and provinces to keep vessels at sea informed of both storms and to advise them to move to safer areas.
"The eye of Storm Haitang is expected to be 16.6 degrees North and 108.3 degrees East at 7am on September 27. Highest wind gusts are forecast to be 62-74km per hour,“ he said.
The storm is forecast to shift west-northwest at 15km per hour over the next 24 hours.
It will hit the mainland before weakening to a low pressure system, with the strongest winds forecast to fall to 39km per hour.
"Haitang, the fourth storm this year, will move over Quang Binh and Quang Tri provinces, bringing heavy rain from Thanh Hoa to Quang Nam and Central Highland provinces," said centre director Le Thanh Hai.
Authorities are providing regular storm bulletins to about 31,500 offshore vessels with more than 147,000 people on board.
A rescue vessel was mobilised but has so far been unsuccessful.
Phat, who doubles as Minister of Agriculture and Rural Development, asked storm-prone areas in the Mekong Delta, particularly An Giang province, to consolidate dyke systems to avoid landslides and promptly harvest rice crops.
Reports from Mekong Delta localities showed that about 385,400ha of rice have already been harvested.