Indeed fergusq, reading the Ca'Non examples that you mentioned, I too can't understand why {vaj} wouldn't be one of the best choices for the Duolingo sentence. I'm just wondering reading SuStel's reply, in case there's something I'm missing.
As I said further on in the Discord conversation, I was thinking
out loud, and concluded that thus and so used in
this way are, indeed, the same part of speech. I wasn't commenting
on the use of vaj, just the use of thus instead of
so.
-- SuStel http://trimboli.name