I showed this thread to a friend of mine who works in the office tower above the shopping mall across the street from the apartment building. He told me that there is a good view of the roof from the 20th floor so I went along the other day to have a look and here's the picture:
I then went down a few floors and fortuitously, there is a view of the elevator shaft through a glass-walled office on the 9th floor:
The building in the background is Saigon Cathedral.
The ladder is not the one shown in Hubert Van Es' famous picture, but interestingly, the diagonal steel I-beam on the roof seems to have been present in the original photo, by the Huey's right skid.