A tourist speedboat overturned about 400 meters from Hon May Rut Ngoai island near Phu Quoc, leaving 15 Indian visitors dead and 21 people rescued.
Thirty-two Indian tourists and four crew were aboard when the boat capsized in rough seas and strong winds; authorities said many victims were trapped inside, complicating the rescue.
Nearby boats pulled passengers from the water before border guards, navy and coast guard teams arrived, while jet skis ferried survivors ashore for first aid and hospital treatment.
Narendra Modi said the tourists were employees of a private mobile phone company attending an annual meeting, and Indian Embassy staff were at the scene as Vietnam investigated the cause.