If you're using SoSci SUrvey with the German user interface, I recommend asking the questions in German, too :)
> My vignettes are text only (no pictures etc.).
Fine. Did you already create 54 texts in the list of questions ("Fragenkatalog")?
This will likele be the most straigt-forward approach, although other venues would be possible.
> My vignette universe contains 54 different vignettes.
Yes, but you already wrote that you have only 54 / 3 = 18 decks, i.e., you display one of the decks in each interview, right? So you would only use 18 codes in the random generator, for example:
1 = TX01, TX02, TX03
2 = TX04, TX05, TX06