Hi
you mention hiring car in Vienna- I can really recommend instead using the car train (autoreisezug) from the Austrian railways, you just drive over border to feldkirch and then you can drive yr car onto train (obviously you get out of itand then sit and enjoy train ride/restaurant car etc and then drive your car off in Vienna 6 or so hrs later, when we went was really reasonable to put car on train. You can check it out on oebb.at