The storm was packing winds of 102 kph in the northern area of the East Sea at 7 a.m. and can get stronger as it continues to move west, according the National Center for Hydrometeorological Forecasting.
By 7 a.m. Monday, the storm will be around 580 km east of the Paracel Islands, with maximum winds of 117 kph.
By 7 a.m. Tuesday, it will be 550 km to the northeast of the Paracels, with 133 kph winds, the center said.
The Japan Meteorological Agency said Nalgae will reach its maximum winds of 110 kph by the end of Monday, before turning to the north of China's Hainan Island. The Hong Kong Observatory forecast a similar trajectory, though saying the storm can pack maximum winds of 120 kph.
Vietnamese coastal provinces from Quang Ninh in the north to Binh Thuan in the south central have been asked to follow the storm developments closely to prepare for possible risks.
The Philippines has recorded 45 deaths from Storm Nalgae that caused floods and landslides in southern provinces, the country's disaster agency said on Saturday, Reuters reported.
Nalgae is the seventh storm in the East Sea this year.
Vietnam was hit by three tropical storms over the past month, which triggered flooding and erosion that severely damaged the country's coastline.