Oh, I assumed the museum would be more about the Khmer Rouge. In fact Wikipedia says

> The landmines in Cambodia were placed by different governments and factions (including the Lon Nol, Khmer Rouge, the Heng Samrin and Hun Sen regimes, as well as the Coalition Government of Democratic Kampuchea who, with international support retained the UN seat throughout much of the 1980s) that clashed during the Civil War in Cambodia in the 1970s and 1980s

https://news.ycombinator.com/item?id=14873083

So it's not clear to me that has anything to do with the US presence in Vietnam.