The storm's centre was over waters off Quang Ninh and Haiphong at 4 pm on August 22, with maximum sustained winds of 62-74 kilometres per hour and gusts at level 10. It was moving south-southwest at around five kilometres per hour.
Strong winds were recorded at Bach Long Vi, Co To, Mong Cai, Uong Bi and Phu Lien.
By 4 pm on August 23, the storm is forecast to move slowly south at around three kilometres per hour, remaining over waters north of Bach Long Vi and about 130 kilometres east-southeast of Haiphong.
It could strengthen to level 9 with gusts at level 11 on August 24 while moving east at around five kilometres per hour, before turning north-northeast on August 25 and gradually weakening into a tropical depression.
The Gulf of Tonkin and coastal waters off Quang Ninh and Haiphong are forecast to see winds of level 6-7, rising to level 8 with gusts at level 10 near the storm's centre. Waves could reach two to four metres.
Forecasters warned that rough seas could damage or sink fishing, cargo and tourist vessels and disrupt aquaculture, shipping and tourism activities.
Coastal areas of Quang Ninh and Haiphong could see winds of level 6-7 and gusts of level 8-9, posing risks to roofs, temporary structures, trees, billboards, power supplies and transport.
From Saturday evening through the night of August 24, northeastern Vietnam, southern Phu Tho and Thanh Hoa are forecast to receive 60-120mm of rain, with local totals above 200mm.
Quang Ninh and Haiphong could receive 150-250mm, with some areas exceeding 400mm, while Lang Son and Bac Ninh may see 100-200mm and locally more than 300mm.
The heavy rain could continue beyond August 24, raising the risk of urban flooding, flash floods and landslides in mountainous areas.