E-liquid is available everywhere in Poland and people do vape in bars (though MarcinP is right - in some places it isn't a great idea, just play it by ear).
Some bars also allow real smoking, though they aren't supposed to and some others have smoking rooms.
Related: E-cigs in Krakow?
I'm going to Krakow in November. Does anybody know of any shops that sells electronic cigarettes, preferably V2 cigs?
You will find disposable e-cigs in almost all places that cell cigarettes although you will find most people prefer to buy the refillable ones. If you have one, you will find liquid for it almost everywhere.