I'm pretty sure Boleslaw the Bold crowned himself as the sovereign king in the year 1076, when the kaiser Heinrich was excommunicated by Pope gregory VII and had no power to stop him, but before that, Boleslaw was his direct vassal. I'm not any kind of history expert so correct me if I'm wrong, but Poland should be in similiar situation as Bohemia in the 1066 start. Even if the vassalege lasted only 10 years after the start, Poland should be part of the empire and maybe had some kind of events/decisions that would help it in becoming sovereign state.