The Coast Guard Region 4 Command said the vessel was detected early on March 6 during a patrol about 120 nautical miles east-southeast of Hon Khoai Island in Ca Mau Province.
Officers inspected fishing boat BV 98768 TS and found several irregularities, including no Vessel Monitoring System (VMS) signal, an expired safety certificate and mismatched identification numbers on the bow.
Six crew members were on board, led by captain Vo Van Tieng, born in 1970 and registered in My Phong Ward, Dong Thap Province.
Initial statements from the captain indicated the vessel was carrying more than 50,000 litres of diesel without invoices or documents proving its legal origin.
Authorities have detained the vessel and seized the cargo, escorting it to the Hai Doi 421 port under Flotilla 42 in Nam Can Commune, Ca Mau Province, for further investigation and legal processing.