Coast Guard Region 1 said the vessel, identified as BĐ-996.xx-TS, was intercepted and inspected in the early hours of May 24 during a patrol operation in Ha Tinh waters.
Two crew members were onboard at the time, including a 33-year-old captain from Quang Tri Province.
According to initial statements, the vessel was transporting about 22,000 litres of diesel fuel.
However, the captain and crew failed to provide invoices or paperwork proving the origin of the fuel, and were also unable to present professional certificates and licences required for operating the vessel.
Authorities said the vessel was later escorted to the Hai Doi 102 Port in Ha Tinh Province for further verification and investigation.
The case is being handled in accordance with Vietnamese regulations on the transport and trading of petroleum products.