Tourists from UK, USA, Australia, Japan, Russia, France, Sweden and Switzerland have confirmed the death, and a Vietnam tour guide was also killed, the government said.
Nine other foreign and local six were rescued by other tourist boats in the emerald waters of the bay.
Ha Long Bay is one of the country's top tourist attractions, attracting over five million visitors a year to see the impressive rock formations irregular and many of them spend the night in wooden boats equipped with sleeping cabins.
The survivors said they saw a wooden board on the ship starting around 5:00 a.m. Local time while the vessel was anchored at night, followed by running water flooding the boat and quickly pulling it off the island Titov, Vu Van Thin, chief administrator of the province Quang Ninh.
"Crew members tried to stop water coming in and alerted the tourists who were sleeping, but the water came in and the ship sank quickly," he said. "All 12 people who died were in the cabins."
There were 27 people, including six crew members on board the boat and all have been taken into account, Mr. Delgado said.
The ship, which is owned by Truong Hai, was moored with dozens of cruise ships and other weather conditions were calm at the time of sinking. The bodies of the victims - most of them aged between 20 and 25 - have been sent to Bai Chay Hospital for identification.
"This accident is a very rare and very unfortunate," said Foreign Ministry spokesman Nguyen Phuong Nga. He said local authorities to investigate and tour companies must improve security measures in Ha Long Bay.
Police have launched a criminal investigation into the cause of the accident, which remains unclear.
