One thing to think about is that the Vistula is significantly long.
It stretches from the Baltic near Gdansk through to Grudziadz, down to Bydgoszcz, through Torun on to Włocławek and then to Płock, Warszawa, Sandomierz, Tarnobrzeg, Krakow and finally to Oświęcim (AKA Auschwitz).
The river is quite wide and reasonably deep for much of its length so it's not too unlikely for a coastal boat from the Gdansk area to ply its trade even as far as Krakow and barges are still very much in use in Poland even today where they are often used for the transport of construction materials (e.g. sand, gravel, girders, timber) and plant machinery & generators.
A friend of mine has some photos of some barges in use on the Vistula that might be useful. I'll see if he can find them so I can post them here.
While I don't know about paddlesteamer type vessels, the Polish did (and still have) a large number of leisure boats from before, during and after the Communist era, ranging from the typical tourist around the harbour types to the faster cruise types you see on Russian rivers. However, one thing I seem to recall (although I can't be certain) was that the tug was used for the adventure because it could cope with possible obstructions on the river and so on as well as being able to tow a barge.
While I was trying to find some info about the Vistula, I came across this
website, lots of images of the river and the settlements on its banks.