Türk Tarihi | History of Turks claims that the Ottomans took Poland-Lithuania as a vassal in the late 1570s. Where did they get this from? I searched everywhere but I couldn't find anything.
Now I have done my research and now I understand. Stephen Bathory was elected as king of Poland and grand duke of Lithuania and remained so for 10 years. In the same year he also became the prince of Transylvania, which formed a personal union between the two nations. So he was a vassal and a king at the same time, but the Polish-Lithuanian Commonwealth didn't give allegiance to the Ottoman Sultan. It is like Stephen Bathory was playing as two different rulers.
Credit goes to TotalMongot for his comment
The Ottomans never conquered Northeastern Europe. The furthest North the Ottomans conquered was Hungary. The Ottomans did attempt to conquer Vienna, Austria, but were defeated in 1689-(with the aid and assistance of Polish mercenary Fighters who were likely fighting on behalf of the Austrian imperial establishment). The failed siege of Vienna, is largely viewed by Historians as the historical turning point for The Ottoman Empire.
Ultimately, it was a combination of (and presence of) the Austrian Hapsburg and Tsarist Russian Empires which served as a geopolitical bulwark against further Ottoman imperial ambitions northward.
(Wikipedia articles on The Ottoman Empire and Austrian Empire).