Typhoon Damrey struck just before Vietnam was set to host the Asia-Pacific Economic Cooperation (APEC) meetings in Danang, which will bring together US President Donald Trump, China's Xi Jinping and Russia's Vladimir Putin, among other leaders.
While Danang itself was spared the worst damage, muddy flood waters have submerged the hardest-hit provinces and swamped nearby Hoi An, a UNESCO World Heritage Site that was set to host APEC spouses later this week.
"We are looking for more than 20 others," the official added.
Authorities said rivers were raging as reservoirs filled up to near capacity in central and southern provinces, with rain still pelting some areas.
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Typhoon Damrey, which made landfall on Saturday, is the latest in a string of major storms to thrash Vietnam's long coastline this year.
Flooding and landslides in northern and central regions killed more than 70 people last month.
The country has reported at least 240 people dead or missing in floods and landslides since the beginning of the year.