So do you think that the at least part of the archduchy of austria (minus say Tirol and the northwestern bits) would be vassalised with the Habsburgs divided into multiple wars and fronts ?
The Habsburg could also move to Prague as the centre of the central european empire.
OK, let me explain an idea more systematically:
1. Soleiman’s offensive is synchronized with the actions of the League of Cognac (which was reasonably successful until Charles mobilized his resources).
2. Instead of concentrating on taking Vienna, Soleiman is just blocking it with a small part of his force and concentrates on doing as much damage as possible to the Austrian lands with the raiding parties going beyond these lands creating enough concern in the HRE. As a side show, some of the light troops can harras the Hapsburg-held Hungary but this is not critically important for the general strategy. The main body of the Ottoman army can advance slowly to provide “support base” for the raiders but without getting bogged down into the sieges (there should be plenty of places without the adequate defenses).
3. The Ottoman (Tatar, Serb, Moldavian, etc.) raiders are causing enough concern within the HRE to force Charles to send Frundsberg & Co against the Ottomans instead of Italy.
4. League of Cognac maintains a numeric advantage in Italy and elsewhere and can grab some disputed or Hapsburg-held territories (Milan would be one of the primary targets).
5. Charles does not have money to pay his troops (as in OTL) and the landsknechts sent against the Ottomans can’t expect too much of a loot. In Italy Spanish troops are not numerous enough to defeat the opponents and also are not being paid. With armies of the League being bigger in numbers and the French still participating, retaining Naples is probably the best case scenario for the Spaniards.
6. Without payment and hope for plunder the landsknechts would rebel sooner or later and Frundsberg’s authority is not absolute. Perhaps the HRE Diet can vote for some subsidies and this may stretch fighting for a season or two but not forever. The problem is that at that time the Western warfare was not up to the task of reconquering Hungary even if some field victories can be scored.
7. From the Ottoman perspective, there is plenty of loot and a chance of moving the “operational base” somewhat to the West making Austria and HRE vulnerable. If they manage to solidify the gain of some territory (say, Eastern part of Austria), they may make it into a vassal “kingdom”, which would cost them nothing even if it is going to be lost sooner or later. The important thing is to “open” the HRE to the future raids.
Of course, the above schema requires certain shift in the Ottoman military mentality but as was demonstrated in OTL later, they were quite capable of using the raiding abilities of their vassals, especially Tatars. OTOH, they still had something of an obsession with the sieges. Of course, the big cities were a potential source of a greater loot but countryside was a good source of the slaves and other goodies and garrisons of the fortified places were usually not big enough to “project power” too far beyond the city walls.