Storm Pakhar, the first to hit the East Sea this year, developed itself from a tropical pressure system, said the National Centre for Hydro-meteorological Forecasting.
At 1pm on March 30, the eye of the storm was about 320km southeast of southern-central coastal provinces from Ninh Thuan to Binh Thuan, with winds gusting up to 74kph at its centre.
The storm is forecast to move west-northwest over the next 24-48 hours at 3-5kmh.
Thunderstorms and heavy rains are likely in territorial waters around the Truong Sa (Spratly) Archipelago and off the coast of Binh Thuan and Ninh Thuan.
Hai also said that priority should be given to accounting for all fishing vessels, and instructing them to remain at anchor.
Authorities were told to prepare to evacuate people from high-risk areas.
Hai also noted that a cold spell forecast to hit northern and central provinces from tonight might complicate the situation
In Hanoi, the temperature will fall 4-5 degrees Celsius to 17-23C, with showers and thunderstorms.
Border guards have instructed eight offshore vessels from the central province of Khanh Hoa to shelter in Malaysia until the storm passes.
Irrigation projects and reservoirs are also at risk in areas to be affected by the storm.
Around 71,000ha or 40 percent of the winter-spring rice crop has been harvested in coastal southern-central provinces. Meanwhile, the Central Committee for Flood and Storm Control sent an urgent message to local authorities and the ministries of National Defence, Transport, and Foreign Affairs.
In the message, the committee asked local agencies to keep fishermen and ship owners informed of the storm's position and development.
Nguyen Ngoc Hai, head of the Binh Thuan Provincial Committee for Flood and Storm Control said that the province was ready to cope with Pakhar.
Around 1,700 offshore vessels had been informed of the storm, and were on their way to safe shelter, he said.