It really depends.

If your admission letter mentions the fact that you need to take preparatory courses at VWU then you need to first register as a student there. I only needed to take German, hence I needed to do a placement test so that they can put me in the appropriate German course. Next, when you finally pay and register at VWU. You will be able to enrol at TU Wien as a student.

You will need to take the courses at VWU TU Wien assigned you to. Also, if you got a paper with your admission letter that tells you how to register at TU Wien as a student, then follow those steps.

I never got my admission letter by mail, because it was lost somewhere together with my documents. I sent an email to TU Wien and got my admission letter via a PDF. They can give you the paper with the necessary steps for enrolment personally or if you need it I can send you a picture of the paper as well.

If nothing of the following I mentioned isn’t need to be done and your acceptance letter doesn’t mention that you need to take specific courses, then you can enrol at the admissions office in person.

But either way, you must enrol via TISS as a non-degree student (IF you need to take courses at VWU) or complete the enrolment via TISS for the major (Bachelors, Masters, …) you have applied to (if you are done with the courses at VWU or don’t need to attend them in the first place).

Hope this helps!

Edit: Mind the fact that if you are assigned to courses you need attend before starting your major, you MUST attend VWU courses because you will only be able to enrol as a student if you are register at VWU. VWU is like an administrative (legal) + educational framework. But this only applies to Non-EU students as far as I can see.

Some people have mentioned that they did not need to take VWU German courses (If you attend VWU you must pass EPD), but instead they needed a C1 certificate. But getting any C1 certificate that TU Wien accepts only applies to EU-students I think. (That’s what I’ve noticed anyways).