According to Vietnam's Central Meteorological Service, the center of Tropical Storm Toraji at 7 a.m. local time (midnight GMT) was in the northern part of the Gulf of Tonkin, about 125 miles off the Vietnamese coast.
The storm is heading toward the coast at about 10mph, with sustained winds of up to 45mph, and is expected to hit Vietnam Friday if it remains on its current path.
Meteorologists issued a warning of potentially devastating heavy rains, flooding, tornadoes and mudslides.
Tropical storms in the South Pacific gather strength over warm water and frequently hit the region during a season that lasts from early summer to late autumn.
In July 2001, a deadly typhoon with the same name (Toraji) swept across Taiwan, killing at least 200 people and causing damage estimated at more than $100 million.