Captain unable to present invoices and documents proving the legality of 30,000 litres of product, it says.
Captain who bought about 20,000 litres of fuel oil from a vessel and was unable to present documents.
Vessels spotted along border of the Vietnam - Malaysia delimitation line at around 1500 hrs on 17 November.
The wooden hulled vessel BD 1013 was carrying a total 20 m3 of waste oil in two cargo holds, said crew.
Phu Yen 2007 was illegally anchored around the outermost edge of an offshore petroleum project area.
Vessel did not have enough documents proving legal origin of 100,000 litres of DO oil, it said in a statement.
Invoices for the product were inconsistent and there was no certificate of origin for the product, it says.
The domestic oil vessel is in process of being transported from Hai Phong port to the May Chai ward, updates.
Captain of QB 91668 TS could not produce any invoices and documents proving the legality of oil cargo.
Captains could not produce invoices and documents proving the legality of a total 100 kilolitres of diesel oil.
