The Communist state won praise for stubbing out the virus early with strict restrictions on movement, extensive quarantine measures and a robust track and trace regime.
It had not recorded a locally transmitted infection for nearly 100 days.
But that changed over the weekend, when a man tested positive for COVID-19 in the southern city of Danang, a beach resort usually packed with tourists.
Since then nearly 100 people who had travelled to Danang have tested positive across the country, leaving health authorities scrambling to snuff out the outbreak.
The number of “infected people increased at a record figure with new 45 cases” the Suc Khoe Doi Song newspaper, the mouthpiece of the Ministry of Health, said on Friday.
The patients are aged between 27 and 87 and all are patients or employed at Danang’s hospitals or their relatives, it added.
State media said a makeshift hospital is being set up at a Danang sports centre in anticipation of the outbreak worsening.
Flights in and out of Danang and public transport within the city have already been suspended, and the majority of its 1.1 million inhabitants have been advised to remain at home.
At least 21,000 people are undergoing rapid test swabs for the virus in the capital Hanoi after travelling to Danang. Similar tests are also conducted in business hub Ho Chi Minh City for returnees from Danang.
Bars have been closed in the nearby Hue, while tourism is suspended in the historic city of Hoi An.
The communist state has won praise for its handling of the virus — reporting just 509 infections in total, including those from the new wave, and no deaths despite sharing a long border with China.