At 7 a.m. Monday it was centered west of the Paracels and packed wind speeds of 90-120 kph, according to the National Center for Hydro-Meteorological Forecasting.
In the previous seven hours it changed direction and began heading north-northeast, and by Monday night is expected to be 280km west of the Paracels with winds of 75-100 kph.
The storm is not expected to hit the islands but will keep moving in that direction, gradually weakening. By 7 a.m. Tuesday it is expected to lie 220 km south of Hong Kong with winds of 60-75 kph.
On Monday central provinces from Thua Thien Hue to Binh Dinh are expected to be hit by strong winds of up to 120-130 kph along the coast and waves two to four meters high.
Meteorological stations in Japan and the U.S. also forecast the storm to weaken by Tuesday.
Rai, the ninth storm to form over the South China Sea this year, was a super typhoon when it hit the Philippines before it started to weaken.
But on Sunday it intensified again into a super typhoon, belying forecasts it would continue to weaken as it moved closer to the Vietnamese coast.
It sank five fishing vessels off the south-central Binh Thuan Province on Sunday, killing one fisherman.
In the Philippines, the death toll from the typhoon, the strongest to hit the country this year, is 208, AFP quoted the national police as saying Monday.