Officers in the mountainous district of Muong Lat, which borders Laos, said they acted on a tip-off and found Ho Khua Dua, 55, acting suspiciously, according to local media reports.
When they asked him to stop to check, he tried to flee.
The officers gave chase and caught him and found him possessing the opium.
He admitted on questioning that he bought the drugs in Laos and was planning to sell them in Thanh Hoa.
Vietnam is a key trafficking hub for narcotics in and around the “Golden Triangle” comprising the mountains of Myanmar, Laos and Thailand, one of the main drug producing areas in Asia.
On October 26 police shot and killed a 30-year-old Lao man in a midnight shootout with drug traffickers in neighboring Nghe An Province.
Drug trafficking and consumption has persisted and grown despite Vietnam having some of the world’s toughest drug laws.
Those convicted of possessing or smuggling more than 600 grams of heroin or more than 2.5 kilograms of methamphetamines face the death penalty.
The smuggling of five kilograms or more of opium resin is also punishable by death.