As long as you study at the University of Vienna, you have to timely pay the students' union fee and possibly the tuition fee every semester.
The tuition fee depends on your citizenship. If you are a citizen from an EU/EEA country, it also depends on the duration of your studies.
If you have paid your tuition fee/students' union fee at another university, you have to report your continuation at the University of Vienna - this does not happen automatically. If you forget to report the continuation of your studies, you will lose your admission.
Information regarding the amount of the tuition fee/students' union fee