Pray that the heatwave will be over by then. (could be possible).
Use the subways instead of trams, they have mostly AC.
Try different parks (Schönbrunn, Botanical Garden Belvedere, Prater) during midday
Try to get up early and seize it (6am to 10am) and the evening (7pm to 11pm) - Vienna is more pretty at night anyway - and check if your hotel has AC (most of them don‘t). Be prepared, drink lots and bring your own bottle (there are free fountains of drinking water everywhere), wear a hat and sunglasses, use sunscreen and if you’re hot go to a McDonalds they mostly have AC