O mais próximo que pude encontrar sobre o assunto foi uma discussão em um dos fóruns . Com a melhor resposta sendo:
As far as I know they are important places in terms of military and national defence; in the border regions as Ben-E said above, offshore, and islands. For instance, some districts located in Phu Tho, Son La, and Yen Bai provinces; Vinh Thuc island in Mong Cai district, Quang Ninh province; Bach Long Vi island in Hai Phong City, and some islands in Thanh Hoa and Nghe An provinces and so on. In some areas, foreigners are permitted to access to for the purpose of specific business operations and a local authority official is assigned to accompany. In some of these places, even Vietnamese people are not allowed to enter as a tourist unless permission is obtained or in cases with the company of functional local authority.
A maioria dos outros são apenas notícias e coisas do gênero:
A melhor generalização é do Reino Unido: