The bus was carrying a group of 40 former 12th graders getting together to celebrate 30 years of graduation from Quang Binh's Dong Hoi High School with a trip to the famous caves.
The accident took place at 9:53 a.m and about 10 km away from the tourist hotspot.
Tran Quang Vu, chairman of Bo Trach District People's Committee, where the tragedy occurred, said nine people died on the spot and six others succumbed to their injuries at the hospital. Of the dozens under treatment, four are in critical condition.
Some local officials said that the driver had possibly lost control when making a turn.
Tran Van Trung, Party secretary of Phong Nha Town, said that the coach had a license plate from Quang Binh Province.
The accident happened in an area with low telephone reception, hindering rescue efforts. While rescue operations are on, authorities have informed the families of the victims.
Deputy Prime Minister Truong Hoa Binh has instructed Quang Binh to mobilize maximum forces and human resources for the rescue effort.
Police said they are investigating the accident and trying to establish if the driver had consumed alcohol or drugs.
Further details were not immediately available.