[aleph_minus_one]

> But does it take 3 years or 4 years to get the engineering degree, after completing vocational training?

You will have the ordinary amount of time at the university.

As I wrote: the time that you save is getting "(fachgebundenes) Abitur" via evening school, which was historically necessary. So you do save time, just not during attending the university.

In the vocational training, one learns practical skills, while the university is there to teach you the theoretical grounding. So, basically nothing that you learned in your vocational training transfers to university. There is a reason why vocational training vs university were historically two completely different and separated kinds of tertiary education.