Vienna was home to the Jugendstil - the most influencial style of early modernism. This is best protrayed by Otto Wagner and Adam Loos. Some of their monuments are included in the Vienna nomination but others - like Wagner-Kirche and Wagner-Haus are not. Later manifestations of influential modern architecture include Wotruba's Cubistic church and three monuments by Hundertwasser (a residential house, a museum and a waste recycling factory) - all of these are outside of Vienna's centre and not included in current nominations.

Like Assif has already said a few buildings are already inscribed as part of the Historic centre of Vienna so perhaps it would make more sense as an extension of that site, rather than a new addition to the list. It would also reflect the continuation of cultural development in Vienna, instead of drawing a line at the early 20th century.
If you are in Vienna then many of the works are well worth hunting out, we had great fun at the Hundertwasserhaus.