Truong Thi Viet, 9, was washed away while making her way home from school in Thang Binh District on Tuesday evening.
Thursday afternoon Bach Dang, Nguyen Thai Hoc streets and a park in Hoi An were underwater and Cam Kim Commune was cut off.
Hoi An Relic Preservation Center began dismantling some ruins in danger of collapsing. Fearing more flooding, the center asked 59 owners of other ancient structures to fortify their properties.
Meanwhile, the town administration sent 250 rescue rafts and VND5 billion (US$294,550) to its 13 wards and communes in preparation for more flooding.
The town government also advised residents - especially 777 families in old houses and in low-lying areas - to evacuate if endangered.
The town's Cua Dai Ward has prepared sandbags to protect houses in case of further flooding and around 100 young people of the ward have joined rescue forces.
The authorities are worried because dozens of boats are still carrying tourists on the Hoai river without life jackets.
By Thursday afternoon water on the Thu Bon and Vu Gia rivers had risen to dangerous levels.
Meanwhile, water pouring down from the mountains has damaged 20 meters of the concrete canal that feeds into Duy Son 2 hydropower plant in Duy Xuyen District. The water is threatening to break the remaining 60 meters of the canal.
The collapse of a bridge several days ago has caused traffic chaos in a number of communes in Dien Ban, Bac Tra My and Nam Tra My districts.