Colonel Nguyen Xuan Son, deputy political commissar of the Gia Lai Military Command, said on Wednesday that personnel and equipment had been deployed to retrieve the object from coastal waters off Quy Nhon.
At around 3 pm, as the tide receded, dozens of military personnel joined relevant units in the salvage effort, using cranes and excavators to dig sand from around the partially exposed object.
Several artillery shells, firearms and other items were discovered near the site during the operation.
The object had yet to be brought ashore by 5 pm despite hours of excavation.
Son said around 40 officers and soldiers had been mobilised for the recovery, which is being carried out at low tide.
Once recovered, the object will be taken to a repair station under the provincial military command for inspection and assessment before further action is decided.
Military officials said efforts were being accelerated to complete the recovery while tide conditions remain favourable.
Authorities said the object may be the remains of a tank abandoned by former South Vietnamese forces during a sea retreat. Similar parts recovered in the area were too badly damaged for display.