The 38-year-old and his wife were arrested on Tuesday when authorities stormed his workshop and found numerous pen guns and ammunition. The couple sold the guns to local gangsters and used the proceeds to feed their drug addiction.
Police found and confiscated five pen guns, 45 rounds of ammunition and equipment used to produce the weapons.
According to police, Nhat has two previous convictions - both for vandalizing telecom devices.
Sentenced to 14 years in prison for the original offences, he was grated parole after being diagnosed with HIV.
Gun laws in Vietnam are extremely strict as anyone caught with one is immediately sent to prison. The only exception being licensed shotguns used for hunting.