Yeah, Stefan the Great was indeed a great ruler of Moldova in the middle of the 15th century. He managed to preserve the independence of his land from the Ottomans by paying annual taxes, but did not really care about Vienna.

Ah, the favourite Vlad Tepes. He was only 17 when attacked his own land (Wallachia) as the head of lent Turkish army... On the throne of Wallachia he managed to massacre some smaller Ottoman outpost, than to avoid the direct conflict with the army of Mehmed II but after that he was expelled by his own brother, Radu, who attacked Vlad also with a huge Turkish army. In the end in the jail of King Matthew he could only impale rats and insects...

Mihai the brave hustled at the end of the 16th century. Buda had been taken by the Ottomans so they were essentially stationing in front of the gates of Vienna. As the ruler of Wallachia, Mihai was a vassal of Zsigmond Bathory (prince of Transylvania), and their united army defeated some Ottoman troops at the Battle of Giurgevo. After that as far is i know Mihai mainly dealt with his personal affairs, rather than fighting against the Muslim oppressors...

The Ottoman forces tried to attack Vienna several times, but they were never able to seize her. The last and maybe the biggest effort took place in 1683 afaik, called the Battle of Vienna. The defenders consisted of Austrian, Bohemian and German soldiers, while the Ottomans had an army of about 100000 warrior including 15-20000 soldier of Moldova and Wallachia...