A list of well rated hotels in the conference areas (central station and Getreidemarkt Campus of TU Vienna) you find here.
Links provided next to the hotel name allow direct booking with the hotel / hotel group. Apart from Andaz am Belvedere, no group rates have been agreed - current price offers for May / June 2021 are quite favourable. It can be expected that prices will be higher in spring 2021.
A brochure of the conference facilities you find here.