They raided the house of Nguyen Minh Nguyen, 24, last Wednesday and found dozens of sacks of used masks.
Nguyen told the police he had got them from Phuc Yen Town in Vinh Phuc Province, 40 km from Hanoi, and stored the masks in his house.
Vinh Phuc is home to Son Loi Commune which has been in lockdown since Thursday last week as eight residents had contracted the new coronavirus.
The demand for masks has been rising since the government declared the Covid-19 outbreak an epidemic on February 1. Many drugstores were fined for hiking the face mask prices amidst the situation.
Vietnam has recorded 16 Covid-19 infection cases so far, of whom 15 have recovered and been discharged.
Vinh Phuc, which accounted for most of the 16 cases, announced its last infection on February 13, the 50-year-old father of a 23-year-old female worker returning from Wuhan. Her mother, sister, cousin, and neighbor were also infected. The man is the last patient in the country still in hospital with Covid-19.
The Ministry of Health can declare a locality free of the epidemic if there are no new cases for 28 days and preventive measures against its spread have been properly deployed.
The global death toll has hit 2,400, and the number of infection cases topped 78,000, mostly in China.