Ok, heres a tip for you. Its a little gamey, but I dont think its unreasonable.
If you haven't already you will eventually be given a mission to recapture Warsaw (which will be put into Swedish hands). You'll need to gather all the lords you can and siege Warsaw. Taking the city can be very, very tough. I waited until there was no Swedish lord int he city, then selected "have a meeting with the fortress commander" and bribed him 20,000 to let me through the gate. After that it was much easier.
When you take the city, choose to ask for it as a reward. Since the King hasn't been giving you anything, he probably wont give you Warsaw either. When he decides not to, rebel and keep Warsaw. Then, go find the King (who will try to attack you) and talk to him- select the dialogue for "we recaptured the city!" to complete the Warsaw quest. You will be sworn in (again) as a vassal of Poland, and you will keep control of Warsaw.