Tropical storm Vamco was over the central South China Sea and centred about 350 kilometres off Danang, Vietnam, with maximum sustained winds of 65km/hour, the Meteorological Department announced at 5am Monday.
The storm was moving in a westerly direction at 12km/hr and was forecast to make landfall over central Vietnam on Sept 15-16.
It was expected to have a significant impact on Thailand on Sept 16-17.
People travelling to central Vietnam were advised to stay tuned for weather updates, the department said.
Locally, a monsoon trough lies across the North, upper Central and lower Northeast and a moderate monsoon prevails over the Andaman Sea, the South and the Gulf of Thailand, the weather office said. More rain was expected in the Central region, the East and the South. Waves were expected to be about 2 metres high in the upper Andaman Sea.