Authorities in Lam Dong Province continued searching for the missing fishermen on October 22 after a naval vessel reached the stricken boat.
Divers found its hull broken and no one on board, raising fears that the crew had drifted away from the sinking vessel.
The fishing boat, BTh 80712 TS, was captained by Nguyen Van Hai, 44, from Mui Ne Ward. The 12.3-metre vessel, carrying four fishermen, had departed for a fishing trip early on October 19.
Border guards were alerted on October 21 that the boat had sunk about 43 nautical miles southeast of Mui Ne Bay, near the Su Tu Trang platform.
Later that evening, the Hai Duong 79 vessel spotted the fishing boat partially submerged, with only its bow above water. No crew members were seen nearby.
Winds were blowing from the northeast, with waves reaching 1.5-2.5 metres.
A regional search-and-rescue vessel has since joined naval and other forces in the search for the four missing fishermen.