Dao Duy Tung, 32, illegally traveled to Laos in March to look for a job, then illegally traveled back to Vietnam on April 22. He was later confirmed infected with the coronavirus on May 6.
Authorities accused him of moving around Hai Duong, Hai Phong, Hanoi, Hung Yen and Ha Nam after returning to Vietnam, making contact with many people.
Four people associated with Tung were found infected afterwards.
Police in Hai Duong said during contact tracing efforts, Tung did not fully cooperate with authorities regarding medical declarations.
He is now being quarantined and treated at the Hai Duong National Hospital for Tropical Diseases.
Everyone entering Vietnam from abroad is quarantined for 14 days and tested at least twice. Illegal entrants are those evading official border checkpoints to avoid quarantine.