The typhoon may trigger downpours, floods and landslides in the central region.
Typhoon Tokage entered the East Sea, internationally referred to as the South China Sea, on Saturday and is moving toward central Vietnam.
It is the ninth typhoon to form in the region this year.
At around 7 a.m. on Sunday, Typhoon Tokage was about 550km southeast off Hoang Sa (Paracel Islands), gusting speeds of up to 100 kilometers per hour.
The typhoon is expected to be about 240km southeast of the Hoang Sa islands on Tuesday and then move southwest before scaling down to a low pressure zone.
It is forecast to bring heavy rains to the central region from Da Nang City to Khanh Hoa Province from Wednesday to Friday, which may trigger floods and landslides.
Vietnam is hit by an average of eight to 10 tropical storms between July and October every year, which often cause heavy material and human losses.