Historically the Ottomans actually got quite close to taking Vienna. After two months the Ottomans had forced thee Austrians to fall back to the inner city and were close to breaching that as well. However, as we all know a strong relief force defeated the Ottomans before that could happen. But what if it didn't? What if the Poles who made a significant portion of the relief force were otherwise engaged, say at war with Russia over Ukraine or Sweden over Livonia. Without them could the Ottomans have taken Vienna or would a weaker, solely German relief force still have been able to defeat the Ottomans?
Assuming Vienna does fall, the Ottomans intended to give it to Imre Thokoly to create a strong buffer against Germany. However, the Holy Roman Empire would never accept the loss of Vienna, so presumably the war would go on. Considering the Ottomans were at the end of their supply chain it its doubtful they could hold Vienna for long. But after Vienna is retaken would the Holy Roman Empire (still lacking the aid of Poland) be able to mount a similar counteroffensive as OTL that takes all of Hungry and much of Serbia. Or would the Turks be able to defeat such a counteroffensive or at the very least would they be able to not lose so much?