A riddle: Which is not working–sites.php or my vhost setup?

I am not an Apache expert. But my setup should work, as it is really straight-forward. It doesn’t and I have ready every post on the internet relating to it. What am I doing wrong?
Please read http://stackoverflow.com/questions/41903013/a-riddle-which-is-not-workin…

There must be another switch somewhere that needs to be flicked. I just don’t find the instructions for sites.php very instructive.

Drupal version: 


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