The tropical depression is forecast to move west-northwest on August 11, with a speed of 20km/h.
The northern region and the north-central provinces from Thanh Hoa to Nghe An are expected to experience downpours on the day.
In response to storm impacts, Vietnamese airlines have adjusted operation plans of flights to and from some affected airports from August 11.
Accordingly, Vietnam Airlines will cancel two flights between the northern port city of Hai Phong and Ho Chi Minh City. It will also delay flights between Hai Phong and Buon Me Thuot, Cam Ranh, Da Lat and Ho Chi Minh City on August 11.
Bamboo Airways, meanwhile, adjusted schedules to ensure absolute safety for flights from/to the Pleiku, Da Nang, Cat Bi, Vinh, and Van Don airports on August 11.
Vietjet also announced that it will adjust schedules of some flights to Hai Phong the same day.
Mulan is the second storm in Vietnamese waters this year. The first, Chaba, arrived in July.