"Heavy rain hit several provinces. Thousands of houses were flooded," said Nguyen Van Hung, an official from the flood and storm committee in the central city of Danang.
The 12 people were killed, and 11 were injured, in the central provinces of Quang Ngai, Thua Thien-Hue and Quang Nam.
A Filipino expert working at Bong Mieu gold mine in Quang Ngai province was missing yesterday, Hung added.
"The typhoon has started to weaken and we are now focusing on dealing with floods," he said adding it was currently moving along the coast and heading north. It was unclear whether it would hit land.
State media said provinces were instructed to issue an alert for flash floods and landslides as the water level was expected to rise.
At least 11 fishing boats sank or were swept away but no casualties were reported among the fishermen.
Thousands of hectares of farmland have been destroyed and traffic was disrupted in the region.