I say if because I do believe it unreasonable. Vienna is an annoyingly hard city to capture historically,
More accurately, Vienna has been very lucky.
the Turks couldn't do it,
The first time the Turks failed because they began their campaign too late and prosecuted it half-heartily. Oh, and don't forget the outbreak of plague in the Ottoman army.
The second time, Vienna was saved by Sobieski and the greatest calvary charge in history. Remember, when Sobieski arrived, Ottoman sappers were in the process of blasting holes in the walls of Vienna. There about 10 000 defenders against about 125 000 Ottomans. Had Sobieski been delayed, Vienna would have fallen.
Further, had Kara Mustafa Pasha been more interested in defeating the Austrians rather than securing the riches for himself, he could have launched an early assault. Vienna would have fallen while Sobieski was gathering his troops. Instead, he choose to hold his troops back rather than let them plunder Vienna.
the Sun King couldn't do it.
The Franco-Bavarian force failed in the WotSS because Leopold I was able to rally the whole of the HRE (save Bavaria) to his defense. Don't forget to include support from Britain, the United Provinces, Portugal, and Savoy. Without the Duke of Marlborough, it seems likely that Vienna would have fallen.
You see, it is not that Vienna is intrinsically hard to take. It just happens that through out history, many different powers have seen it to be in their best interest to protect Vienna.
In the WotAS, Marie Theresa did not have this support. Prussia, Bavaria, and Saxony were aligned against her. Hanover had declared its neutrality. Britain was extremely hesitant after the embarrassment of Cartagena. There was no Sobieski or Marlborough riding to the rescue.
In fact I think only the Hungarians ever did it.
And Napoleon (twice, I think). The Soviets too. Prussia would have taken it in 1866, the Habsburgs surrendered too quickly.
Yea Bavaria had France and Prussia, whoop-de-freakin-do, its still a pain in the ass to have to seige a city and have the closest route for supplies from home run through the Alps or occupied Territories and thats just from Bavaria, Prussia and France are a whole other story.
Perhaps you should read up on the situation during 1741, especially September. Upper Austria was 'occupied' by Bavaria. More accurately, Upper Austria had received Charles VII rather warmly. There was no chance of partisan action to threaten his supply lines. He had ~20 000 troops in Linz. Another 20-40 000 from France and 15 000 from Saxony were moving to join them. There were virtually no troops in Vienna. Austria's forces were tied down defending the Bohemian lands or withdrawing to Pressburg to join Marie Theresa and her court. There was little to stop the fall of Vienna.
So, why didn't Vienna fall? Two factors stand out: Frederick II cut a side deal with Marie Theresa, and Charles VII decided to take Prague instead. (He was afraid that Frederick Augustus would beat him to it.) In the world of AH, either or both of the events can easily be changed.