Where do you need cash to pay for items in Wien eg. supermarkets, take out stands, metro ,sit down coffee/dessert shops ??
You can pay virtually everywhere with your credit/debit card or NFC-capable mobile phone (e.g., ApplePay).
The number of exceptions, i.e., cash-only places, is constantly diminishing (other than in Germany). Supermarkets are no problem, most of the take out stands also, as well as restaurants and cafés. Very old and traditional Viennese pubs (called Beisl) insist on cash, but normally this is clearly communicated. Even at modern public restrooms cashless payment is provided.
For public transport it is expected that you have already a ticket when boarding or for the U-Bahn (metro, subway) when crossing the colored bars in the station. There are no turnstiles or gates, and you need not to show your ticket. But when you are checked, this happens suddenly and randomly, and you do not have a ticket there is no mercy and no excuses are accepted. The fine is €135 upfront.
Best is to buy a public transport pass online (via a PC or mobile phone app), most common is the "7 Days Vienna digital" for about €25, giving you unlimited access to all means of public transport, day and night, within the area of the State of Vienna.