Short answer, it didn't. The country was divided into four roughly equal occupation zones. And Vienna was also divided into four zones, just like Berlin. It stayed divided until late 1955 when the occupation forces left.

Austria had several advantages over Germany that brought the division to an end. First was an agreement that Austria was to be treated as a victim of Nazi expansion, rather than willing collaborationists. So unlike Germany there were no major voices pushing for dismemberment, and their borders were to be preserved.

The second advantage is that the Soviets weren't particularly concerned about Austria. It was far enough west it didn't fall in their required zone of influence. They wanted a series of buffer states and Austria didn't directly border the USSR. As long as the USSR kept Hungary and Czechoslovakia in their sphere, Austria wasn't necessary for defense, especially once Tito and Yugoslavia broke with the USSR. Austria would be strategically isolated and a potential liability to defend against.

Finally, the Austrians themselves pushed hard to negotiate an end to occupation. When the Austrian chancellor Raab traveled to Moscow, he was able to negotiate independence by promising payments to pay for damage done by Austrians during the war and a promise of neutrality on the Swiss model. The Soviets accepted this, and the West fearing a division similar to Germany jumped at the chance to rubber stamp the Moscow agreement. The neutrality clause was written into the constitution and is the reason Austria isn't in NATO today.