What happens if the Ottoman siege of Vienna in 1529 is successful?
It would be known as the "Battle of Vienna" rather than "Siege", for one, given there isen't time to reduce the fortress before the campaign season is over. The Ottomans are too far away from a stable secure base to go into winter quarters, so the only way they can take the city is by storm.
Now, the direct military effect is likely minimal: the Austrians would have to aknowledge Ottoman rule over Hungary, probably see a tribute payment raise, and maybe demolish some fortresses. Far larger, though, would be the prestigious/religious effects. Here's the nominal August Emperor of the Romans, crowned by the Pope and the Church's declared champion of Christianity on earth, forced to flee his stronghold in the face of the Muslim Hordes. What greater proof that the corruption charges against the Catholic Church are true enough that God has withdrawn his favor and protection from them? Protestant theoligans and rulers, with the movement just starting to get political traction, would be given a huge confidence and legitimacy boost, which could easily feed into a wider and earlier spread of the ideas.