ShotHotspot has mapped 15 abandoned places in Vietnam, ranked by how often they're photographed, viewed and voted for. The most popular are Banksy Saigon Same Old, Jungle Temples Angkor Buddhist and Hanoi Flats Dinh 20160329. Some are private property or unsafe, so get permission before you visit.
We don't know whether you're allowed to visit any of these places. Many are private property, closed to the public or unsafe: get permission from the owner or the relevant authority, and take safety precautions.