Only 40 minutes after it left a port in Singapore, the 5,900-dwt Sunrise 689 (built 2012) disappeared last Thursday.
It later emerged a group of more than 10 pirates destroyed the communication and navigation systems and kept all 18 crew as hostages.
Crew members said the pirates put knives to their throats and threatened to kill them.
Reuters reported that two of them were injured, quoting a manager of Haiphong Sea Product, owner of the vessel.
It was carrying more than 5,000 tonnes of gasoil and was heading to Vietnam.
Vietnam's coast guard was escorting Sunrise 689 to Phu Quoc island, off the country's southwestern coast.
At least 11 vessels have been hijacked in the Strait of Malacca or off south China since April, according to the International Maritime Bureau's piracy reporting centre in Kuala Lumpur.
