The Soviets aren't going to occupy all of Austria. By the Vienna Offensive the post-war order had already been mostly determined. They could however impose a situation like in Germany, where East Austria (which was occupied by the Soviets) becomes its own state.
Simply we need a few things to occur.
1.) Austria is treated by the allied powers not as a state invaded by Germany, as the Red-White-Red book claimed, but a fully allied and thus liable power.
2.) USSR-Yugoslavia split remains thus creating the continued need to have forces to invade the north of the state.
3.) Informbiro period never occurs at all, or merely is a cool affair of mutual disdain. Which would be Stalin deciding to keep expanding his borders to secure a "friendly" communist state.
4.) Stalin in the moment of occupation openly calls for a Austrian people republic instead of adhering to the occupation as per OTL.
Lots of options are present but a lot of this hinges on the Allied powers treating Austria as an occupied enemy rather then a state forced into joining Germany. Perhaps something as simple as 13% of the SS was Austrian.