I had to check and apparently a vasistas is originally a transom windows and I’ve one on my house front door. It’s the window panel there is on some doors with worked iron on the other side that you can open but won’t allow people from outside to go in. Historically, people didn’t open the full door when people came to their house, just the window part and German would say was ist das?. And when modern velux windows become popular, they were also nicknamed vasistas by older people for some reason? None of this makes sense.