A passing-by ferry discovered the sunk boat, Hoang Phuc, with 16 fishermen on board, and tried to rescued them but they could only help 11.
Director of the Vung Tau's Maritime Search and Rescue Co-ordination Centre, Region No 3, Luong Truong Phi, said the centre was mobilising all their forces and equipment to search for the remaining five missing fishermen.
A rescue ship from the nearby Can Gio border guard has also been sent to the scene to take part in the rescue.
VNS