Dave Fetty, our master glass artisan at Fenton Art Glass in West Virginia USA, has made similar pieces since his experience here with Robert Barber in the mid-1970s. We call them "window" vases, and I see that this eBay seller uses the German word "Fenster" in the description (my knowledge of German is 40+ years old, so don't ask me to translate the whole thing!).
I've watched Dave make these window vases quite often and would be glad to offer a step-by-step explanation if there is interest in such.