As stated, there's a long history of rainwater leaks across the VAG group over many years. I've yet to own one that hasn't leaked after a few years- current Skoda excepted-yet! That doesn't make it right of course and your car should stay dry. If there's excessive condensation, you have a leak and your car is brand new under warranty. The salesman is fobbing you off, irritating but fairly typical. Normally, water will pass down between the window and the window rubber, then drain out the bottom-normal. Sometimes the drainage holes at the base of the door can be blocked with paint or underseal-- need to check that. But in my experience the doors tend to leak through the door cards. Inside your door trim there's another barrier, which used to be a polythene sheet sealed around the edges. These leak quite badly and the rain water, instead of draining out the bottom, seeps through into the trim. Later cars- and I think--yours-- have a metal sheet instead of plastic, but this is sealed with some type of foam seal. This can and does also leak. There is also the risk of damp getting into the wiring and control modules located inside the doors- not sure if this affects Polo but it does Golf/Octavia. Keep onto VW as above- they need to sort it for you. Good luck!