Aside from the problem with Vienna, the occupying powers didn't just "decide" to divide Germany - it was a process with descions on both sides.
You'd probably have to butterfly the Moscow Declaration before WWII even ended. Then the Austrian Declaration of Independance on 27.4.1945 and more so the acceptance of that Govermnent by the Western Powers in October. It essentially makes it far harder to devide the country with a unified government.
Then you need to look at goals. The Austrian population isn't all that important to either side. The industry in the east has been looted of everythign that could be reasonably moved. That more or less leaves topography. Here Austria is an important north-south route. If the Soviets want to deny this one to the Western Powers they either need to conquer them by force - kicking off WWIII most likely - or by keeping Austria out of the western block, something they felt they managed with the declaration of neutrality in the "Staatsvertrag" in 1955. As far as I know something like that had been the plan for a while in Soviet circles. Not that it worked out quite as intended in the end - look at the Hungarian Crisis or the Prague Spring and the surrounding diplomacy. Still, it kept Austria out of NATO and prevented them from joining the proto-EU that formed at the time.