At 7 a.m. Monday it lay 300 km from China's Leizhou Peninsula packing winds of 133 kph.
The National Center for Hydro-Meteorological Forecasting (NCHMF) said over the next 24 hours it would move north-northwest at 15-25 kph and possibly intensify.
By 4 a.m. Tuesday it is expected to move 190 km from Mong Cai Town in Quang Ninh and 290 km from Hai Phong City with winds of 167-183 kph.
It will continue to move in that direction and enter waters off Quang Ninh and Hai Phong before making landfall and weakening into a tropical depression by Wednesday night.
The Japan Meteorological Station said Monday morning that the storm is set to enter the Leizhou Peninsula area with winds of 126 kph, but would weaken somewhat when it arrives in the Gulf of Tonkin, with the winds decreasing to 108 kph.
The Hong Kong Observatory said the storm would have winds of 140 kph when it crosses Leizhou, would move along the China-Vietnam border in Mong Cai Town of Quang Ninh before moving further towards Cao Bang and Lao Cai provinces and weakening.
The NCHMF said the storm would bring heavy rainfall to the northern region - of 200-400 mm, rising to as high as 500 mm in some areas - between Monday night and Thursday morning.
Coastal localities in the region should expect waves of 5-7 m, it said.
Quang Ninh, Lang Son, Cao Bang, Ha Giang, Lao Cai and Yen Bai provinces should brace for flooding and landslides, it warned.
Prime Minister Pham Minh Chinh on Sunday ordered coastal provinces in the north and north-central regions to prohibit vessels from going out to sea and evacuate people living in places vulnerable to strong winds, landslides and floods.