The Vienna Offensive lasted from April 6th to 10th 1945. The Red Army surrounded the city in early April after moving through Ukraine, Hungary, and Slovakia.
armchairgeneral.com Soviet tanks in Vienna
Vienna was briefly under siege but since few German troops remained in the city at that point, The Red Army was able to capture it relatively quickly. The lack of German troops meant that some areas had heavy resistance and urban combat while there was virtually no fighting in others. The city was firmly under Soviet control less than 2 weeks after the battle began. Some soldiers retreated to Linz and Graz, where they were defeated before the end of April.
histomil.com Soviet forces pose for a photo shortly after the liberation of Vienna in April 1945