The present study surveyed 373 Vietnamese subjects (15–47 years) to find whether they use medical mask properly and find the determinants of this correct behavior. Vietnam is one of the countries that has enforced compulsory medical mask policies in public areas. This study offers insights on citizens’ behaviors and recommends reasonable policies. The study found that only 22.25% of respondents demonstrate proper use of a medical mask based on the World Health Organization recommendations. Moreover, people with higher risk perception are likely to misuse a medical mask, which implies that the panic might lead to a higher risk of transmission when wearing a surgical mask. Older respondents are cautious about proper use of masks, while, those who are cautious about the mask’s characteristics use them fairly properly. This study also suggests policies of public communication and instruction in proper use of a medical mask to contain the COVID-19 outbreak.