All religious, sporting and cultural events will also be suspended. The use of masks in public places will be mandatory, and meetings of over 30 people will be prohibited.
Vietnam keeps its borders closed to international travelers, and those arriving in the country will have to stay in quarantine facilities.
"The new cases may be just the tip of the iceberg. We assume there are many more people infected," Harvard Medical School's Hanoi-based expert Todd Pollack said.
The country was about to reach 100 days without reporting a new local infection. Activities had returned to normal when a 57-year-old man was COVID-19 positive last Saturday.
Local authorities quarantined at least 50 people who had direct contact with him.
Today is the birthday of Vietnam's revolutionary leader, Ho Chi Minh, who was immensely influenced by the struggles of African Americans. During his time in Harlem, he wrote a book called "The Black Race," detailing the horrors suffered by the African-America communities. pic.twitter.com/xeONP1s5dF