In case you only need less than 8 ECTS and/or the master thesis to finish your degree programme you need a confirmation of the director of studies (Studienprogrammleitung, spl.univie.ac.at). The confirmation has to state that you don't need any ECTS (or the number of remaining ECTS) to finish the master programme and that you are currently writing your master thesis.
Within the payment period you have to send the confirmation to the Admission Office (contact form - indicate your u:account e-mail address!). For the next two semesters only the single tuition fee will be prescribed. If you do not finish your master programme within these two semesters, the double tuition fee will be prescribed.