Malaysia’s Maritime Enforcement Agency (MEA) said that the suspects had made the confession to the Vietnamese authorities.
The men were arrested on Friday in Vietnamese waters near Tho Chu Island after they reportedly fled the 7,301-dwt Orkim Harmony (built 2009).
Reports initially said the suspects had claimed that they had come ashore after their fishing vessel had sunk.
State news agency Bernama, quoting Malaysia's naval chief Admiral Abdul Aziz Jaafar, said the pirates had repainted the ship and changed the name to Kim Harmon.
The Malaysian-flagged Orkim Harmony lost communication with its owners in early June.
The ship was reportedly carrying 5,879t of RON95 fuel worth MYR 21m ($5.5m) which belongs to state-owned oil company Petronas.
The ship also had a crew of 22 people made up of Indonesians, Malaysians and Myanmar nationals.
Another five pirates thought to be involved in the hijacking of the Orkim Harmony are reported to be still at large.
It was the second Orkim Ship Management-controlled vessel to have been attacked by pirates in the space of eight days.
The 7,119-dwt Orkim Victory (built 2009) was hijacked and had its cargo of fuel siphoned off and stolen in the early hours of 4 June 2015.
