Which two provinces have never merged since their establishment?

These are two localities that have never undergone a merger or separation process, and have retained their original names from the feudal era to the present day.

Correct

In 1976, Vietnam had 38 provinces and cities, including 35 provinces and 3 centrally-administered cities. This was the year with the fewest provinces and cities in Vietnam in the past half-century.
At that time, Northern Vietnam comprised 13 provinces and 2 cities: 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 , and Vinh Phu. The two cities were Hanoi and Hai Phong.
Central Vietnam comprises 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, and Lam Dong.
Southern Vietnam comprises 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.

Source: https://vtcnews.vn/hai-tinh-nao-chua-tung-sap-nhap-tu-khi-thanh-lap-ar930922.html