Vietnam's long coastline makes it prone to destructive tropical storms. Natural disasters such as floods and landslides triggered by storms killed 389 people in the country last year.
At least 19 people were killed by a tropical storm last week in the southern resort city of Nha Trang.
"(Usagi) will start hitting our operational oil and gas blocks off the central and southern coast and all units must keep vigilant as the storm develops," state-run Vietnam Oil and Gas Group, known as PetroVietnam, said in a statement on Friday.
With sustained wind speeds of up to 100 km per hour (62 miles per hour), the storm will dump up to 500mm (19.7 inches) of rain in coastal areas stretching from the central city of Hue, to the southern province of Binh Thuan and the Central Highlands, the National Centre for Hydro-Meteorological Forecasting said on Friday.
The Central Highlands is Vietnam's largest coffee growing area, where the harvest of the 2018/19 crop year is underway.
"We are concerned that the heavy rains triggered by the storm will disrupt the coffee harvest," said Nguyen Viet Vinh, General Secretary of the Vietnam Coffee and Cocoa Association.
Traders estimate that farmers have harvested nearly 40 percent of the beans and that the harvest will end in late December.
Vietnam's government said on Friday it had ordered local authorities to get ready to evacuate people from areas prone to floods and landslides and instructed boats to find shelter.
Usagi, known in Vietnam as "Storm No. 9", is forecast to weaken to a tropical low pressure system as it hits the southeastern coast of Vietnam on Saturday night, with sustained wind speeds of 60 kph, the meteorological agency said.
(Reporting by Khanh Vu; Editing by James Pearson and Christian Schmollinger)
