Which two provinces have never merged since their establishment?

Correct

In 1976, Vietnam had 38 provinces and cities, including 35 provinces and 3 centrally-run cities. This was the year with the smallest number of provinces and cities in Vietnam in the past half century.
At that time, the North had 13 provinces and 2 cities including: Bac Thai, Cao Lang, Ha Nam Ninh, Ha Bac, Ha Son Binh, Ha Tuyen, Hai Hung, Hoang Lien Son, Lai Chau, Quang Ninh, Son La, Thai Binh , Vinh Phu. The two cities were Hanoi and Hai Phong.
Central region: 10 provinces: Thanh Hoa, Nghe Tinh, Binh Tri Thien, Quang Nam - Da Nang, Nghia Binh, Phu Khanh, Thuan Hai, Gia Lai - Kon Tum, Dak Lak, Lam Dong.
Southern 12 provinces: Song Be, Tay Ninh, Dong Nai, Long An, Dong Thap, An Giang, Tien Giang , Hau Giang, Kien Giang, Ben Tre, Cuu Long, Minh Hai and Ho Chi Minh City.