I couldn't recommend more Nikita's apartment. It's located on the first floor of a building in a very calm and quiet street, but only a few minutes away from all the main museums, 15-20min walk from the city center, and right next to the metro station "Volkstheater" from which you can go anywhere easily.
To enter, you need an app called Nuki to lock/unlock the door. It's very practical, but you have to have a smartphone and make sure to have enough battery.
The place itself was spotless, well organized, felt like a cute, cozy and charming home. You can find anything you need to cook, clean, wash your clothes... with lots of storage. The bed is also very comfortable!
Nikita was a super helpful, flexible and friendly host, sending us all the necessary information and being reactive when we needed help.
All of this made our stay in Vienna perfect, given that we also really fell in love with the city!