Mulan, the second storm to enter Vietnamese waters this year, is forecast to enter the Gulf of Tonkin at 7-9 p.m. Wednesday, bringing heavy rains to the northern region over the next three days.
Quang Ninh, home to Ha Long Bay, and the port city of Hai Phong will be the first localities to be hit by the storm.
Authorities in Ha Long Town ordered all tourist cruise ships on the bay to suspend operation from Wednesday afternoon.
Around 20,000 tourists are now in Ha Long.
In Co To Island, around 80 kilometers from the mainland, Do Huy Thong, vice chairman of the island district said that many tourists had left the island, but there are still more than 260 domestic tourists left.
"District authorities have informed tourists about the storm and boat schedules so they can decide to return to the mainland or stay on the island," Thong said.
Hoang Trung Cuong, vice chairman of Cat Hai District in Hai Phong, said all tourism activities on Cat Ba Island have been suspended.
Around 1,000 tourists, including 81 foreigners, are on the island.
Local authorities advised tourists to leave the island by 4 p.m. Wednesday to avoid being stranded due to the impacts of the storm.
According to the Japan Meteorological Agency, Mulan will move between Leizhou Peninsula and Hainan Island at a speed of 65 kph before entering Quang Ninh Province. The Hong Kong Observatory concurred but expected winds of 85 kph.
Vietnam's national forecasting center said the coast between Quang Ninh and Hai Phong will see winds of 40-60 kph from Wednesday night and through Thursday.
The entire northern region and two north-central provinces of Thanh Hoa and Nghe An will receive rainfall of 100-200 millimeters, and 250 millimeters in some places during the period.