The National Centre for Hydro-Meteorological Forecasting said the storm had intensified and was moving towards waters off Quang Ninh and Hung Yen, with landfall expected near the Quang Ninh-China border late on July 4 or early July 5.
The storm is forecast to bring Category 6-7 winds across the western part of the northern East Sea, including the Hoang Sa (Paracel) Islands, from Friday night, extending into the Gulf of Tonkin. Winds near the storm's centre could reach Category 8 with gusts of up to Category 10, causing very rough seas.
Strong winds are also expected along the coasts of Quang Ninh, Haiphong and Hung Yen from Saturday afternoon.
Heavy rain of 100-200 mm is forecast across northeastern Vietnam and Thanh Hoa from Friday night through Sunday, with 150-250 mm expected in parts of the northeast and isolated areas receiving more than 350 mm.
The Department of Dyke Management and Natural Disaster Prevention and Control advised travellers to monitor weather forecasts before and during their trips and consider postponing travel if conditions worsen.
Tourists were urged to avoid going to sea, boarding boats or speedboats, paddleboarding or swimming during storm warnings. They were also advised against visiting waterfalls or streams, camping near rivers, or trekking in mountainous areas during heavy rain.
The department warned that mountain trails could remain hazardous after heavy rain because of the risk of flash floods and landslides, and advised against trekking alone or using unofficial routes.
Motorists were also warned not to drive through flooded roads or low-water crossings, as even shallow floodwaters can sweep away vehicles.