The police took in Pham Vu Linh while he was sitting at a coffee shop in Long Xuyen Town wearing a sergeant major's uniform last Saturday.
The arrest followed complaints by locals.
Linh has reportedly been spotted wearing a police uniform and acting like an officer more than a few times.
He told the police that a friend had given him the uniform and he had acted like an officer just to show off.
One time he pretended to be an officer and cleared traffic for an ambulance, and some members of the public shot it and admiringly posted it on social media, where many praised him for his action.
He subsequently did the same thing twice more.
The police decided he did not personally benefit from impersonating a police officer and are still investigating to clarify his motives to handle him properly.