To add a bit more info to this I’d say the best advice is to watch out for a sign saying:
Khu Vực Cấm
Normally there is an English translation below but not always. Passing a sign like this is not advised and once got me in a bit of trouble. These are generally for military areas though locals will be able to pass without a problem (when it is a region). In these cases Vietnamese are more likely to ask you to politely leave unless they suspect you are up to no good e.g. drug smuggling, wildlife hunting etc.
Some parks etc. are also restricted and require permits and in some cases some overzealous workers might say a place is restricted because it was in the past even though it no longer is. Local advice is often difficult to find and if you do hope to explore try asking in the nearest major town (Thanh Pho). Tour guides in the tourist cities may give conflicting information if it isn’t on the regular ‘routes’ they sell to the majority.
Closest I could find on the subject was a discussion on one of the forums. With best answer being:
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.
Most others are just news stories and the like:
Best generalization is from the UK:
Credit:stackoverflow.com‘
4 Mar, 2024
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