A fisherman in Thua Thien-Hue captured a seal on Tuesday, and plans to sell the animal to the highest bidder.
Three others have been seen swimming the sea off the central coast - where the species are considered rare.
In general, however, the animals are known to seek refuge in marshes, inlets and lagoons after fighting through heavy storms in the open sea.
The captured seal weighs around 30 kilograms and is being kept in a cage by the fisherman while local authorities attempt to determine whether or not to take action.
Hoang Luong, 38, who caught the seal while fishing in the salty waters of Tam Giang Lagoon in the Huong Tra District.
Luong told Dan Tri newspaper that the seal was accidentally caught in his net 200 meters off the coast of the Hai Duong Commune.
Luong had to ask for help from seven other fishermen to haul the animal in. He said the whole process took nearly half an hour the animal was very strong and attempted to bite them. Luong also said some trader offered to buy the animal for between VND1 million (US$51) and VND3 million, but he refused.
The fisherman is hoping to net a much higher price, he said.
Nguyen Tat Thanh, a local resident, said other local fishermen had reported numerous sightings of the aquatic mammal over the last 20 days.
Nguyen Quang Vinh Binh, head of Thua Thien-Hue Fisheries Protection Agency, said the agency is identifying the species of the seal and will seek advice from higher authorities.
The agency has also offered Luong VND2 million for the animal and threatened him with fines if he fails to cooperate all to no avail.
Binh said his agency has warned fishermen in the region from trying to catch seals.
Simon Faithfull, a technical advisor for Wildlife At Risk says that the seal's appearance strikes him as extremely surprising. A quick scan of a regional book on aquatic mammals in South-East Asia turned up not a single species of seal. He guessed that the creatures had been thrown off course by heavy weather - perhaps the recent typhoon in North Korea.
As a conservation advocate, Faithfull advised that offering to pay for the release of any animal could send the wrong message.
"You don't want this fisherman to get the idea that he can somehow make a living off of catching these animals," he said.