According to the National Center for Hydro-Meteorological Forecasting, Mulan, the second storm to enter Vietnamese waters this year, lay centered 480 km from Mong Cai in the northern Quang Ninh Province at 7 a.m. on Wednesday with winds of up to 62-88 kph.
Over the next 24 hours it would move west-northwest at a speed of 15-20 kph.
It would arrive over the Leizhou Peninsula in China at 5-6 p.m. and enter the Gulf of Tonkin two hours later.
At 7 a.m. on Thursday it would lie 70 km south of Mong Cai, a town on the China border, and 110 km north of Hai Phong port city, with wind speeds of 62-74 kph.
When approaching the northern coast, it would weaken into a tropical depression before becoming a low-pressure area by Thursday.
According to the Japan Meteorological Agency, Mulan will move between the Leizhou Peninsula and Hainan Island at a speed of 65 kph before entering Quang Ninh Province. The Hong Kong Observatory agreed about the path but expected winds of 85 kph.
The coast between Quang Ninh and Hai Phong will have winds of 40-60 kph from Wednesday night and through Thursday.
The entire northern region and the two north-central provinces of Thanh Hoa and Nghe An will receive rainfall of 100-200 mm, and 250 mm in some places during the period.
Heavy rains would continue to batter the north on Friday, the weather office said.
Vietnam should expect 10-12 storms and low-pressure areas in the East Sea this year, with around half making landfall.
The first, Chaba, arrived in July.