The accident occurred at about 12.40 pm near Luon Cave in Halong Bay, according to the Halong Bay-Yen Tu World Heritage Management Board.
The vessel, operated by Halong Bay Services JSC, was transporting tourists from the cruise ship Leona Cruise to visit Hang Luon when it lost control while returning passengers to the ship and collided with a buoy mooring tourist boats outside the cave area.
All 15 people on board fell into the water and were rescued by local emergency responders and company staff.
The boat was carrying 14 passengers and one operator, including tourists from South Korea, India, the Netherlands and Germany, as well as Vietnamese passengers and a local tour guide.
Three people were injured in the incident, including a German tourist, a Vietnamese passenger and the boat operator, authorities said. They were taken to hospital for treatment.
No fatalities were reported.
The vessel has since been recovered and moved to a safe anchorage.
Authorities said preliminary findings indicated the crash was caused by steering failure. An investigation is under way, while officials are assisting victims and processing insurance claims.