Quang Binh province is the hardest-hit of the central coastal provinces with about 40,000 houses submerged.
Most of the deaths were in the Central Highlands coffee belt where flash floods swept people, houses and crops away.
A commune official was quoted by the Tuoi Tre newspaper as saying: "I have been working here for more than 20 years, but this is the first time I saw such frightening floods."
Ha Tinh province reported 13 deaths, including six children. One person was reported missing, said Trinh Nhu Tien, a provincial disaster official.
The government said more than 48,000 homes and 65,700 hectares of crops were under water
The tropical storm was downgraded to a depression on Monday, but heavy rains continued, the national weather centre said
Tropical storms and typhoons often strike Vietnam from August to October. Last year, 10 storms hit the country and about 500 people were killed by floods and landslides, the government said.