But what is the Soviet border? The example -- "Rivers on a winter zone
border, such as the Vistula, do not freeze" -- mentions a river that is in the middle of Poland and is to the west side of East Prussia.
Yeah, I also realized that today, myself. So I looked at a few WWII maps and it appears that for a while the western border of Poland was the Vistula river. So I rationalized that's why it's mentioned in the example of a Winter Zone border river.
But your question I think still stands... the Vistula does run through the middle of Poland on the ViE map. So why is it used as an example of a Winter Zone border river?