We have a flight leaving Vienna Airport at 8:50 AM on a Sunday when Hertz doesn't open until 8 AM. Has anyone returned a car to this location prior to opening? I question whether we can make our flight if we wait until the Hertz counter is open. Is there a drop-box or some other way of handling returns when not yet open for business?

Check with Hertz.
I believe their office in the lower level of the terminal has a drop off box.
The rental firms are mostly all located in this area across from the luggage storage area and near steps to the departure/ check- in hall.