Larger conferences can overwhelm Vienna in terms of hotel availability, and accordingly prices rise dramatically.

I would have suggested the Arcotel Kaiserwasser, right by the Kaisermuehlen U-bahn station (which is the station for the Austria Center, so that is within walking distance from the hotel), but the rates rise from €119 on March 3rd to €300 on March 4th, which is when presumably the conference starts.