If we assume that the Ottomans did manage to conquer all of Hungary following a successful siege in Vienna in 1529, could the Hapsburgs have eventually retaken any of it? Possibly but there is no doubt that they would have been in a significantly weaker position than the ottomans were in this TL.
Personally, I doubt this. At least in the near term. The Ottomans were much more hands-off as rulers than the Hapsburgs were, and it's hard to see them deciding to rule former Hapsburg Hungary directly anyways, given how distant it is from Constantinople. So most likely it is a vassal, or part of an expanded Transylvanian vassal (though most likely it would just be called "the kingdom of Hungary"), and as such, for at least the next century I'd expect the Hungarians to favour the Ottomans over the much too German (and probably still too religiously interventionist) Hapsburgs.
Now, after a century or two, there's plenty of scope for the Hapsburgs to retake Hungary if they've had better fortune than the Ottomans. For example, a plague running through the Ottoman world at a time when the Hapsburgs have managed to unite Germany under their rule or simply have not so many distractions and are thus able to capitalize on the Ottoman weakness... Yeah, that's totally possible even if the Ottomans have otherwise been having an absolute ball and crushing all comers.
The Ottomans taking Vienna would be a major crisis for the Habsburg and would force them to compromise earlier with the Lutherans, including the ones in their own domains. If Charles V have to compromise this early instead of late in his reign, it would be great as he avoid spending his entire life fighting them.
I have difficulty seeing Charles V compromising. I would have expected him to instead double down to ensure that Germany was Catholic and united.
But Ottoman attention being needed for the Persian matter and sheer distance will, in my opinion, minimize the impact on Europe. The Hapsburgs lose Hungary, but they aren't much reduced, since their wealth and power is in Spain, Flanders and Bohemia and they get time to recover themselves. The Ottomans, once the war with Persia is over, are likely going to turn their attention to the Ukrainian Steppes or dominating the Eastern Med (where Venice is their main antagonist). They aren't going to try to conquer Prussia or Spain - both are much too far for their lines of supply to support a serious army at such a distance. Further attacks of Vienna may be on the cards, but again, those aren't going to result in big territorial shifts.
What the victory could mean is that the Hungarian plain and the northern Balkans are more secure against raids and invasion, so are more populous and prosperous. And Hungary in particular could turn into a great refuge for Protestants and Calvinists from the religious strife in Catholic Europe. I would expect that to mainly be driven by the vassal kings and major land owners of Hungary, but we may occasionally see the Ottoman Sultan get involved a little. The Sultan being the protector of the Protestants in Central Europe would be... Interesting.
fasquardon
