is the pronunciation of "vegetariano" (vegetarJano etc) a regional dialect or is there some rule to tip me off that the "ia" is pronounced hard J?
Yup, they use some odd dialects. Just had "bebo" pronounced bevo with a hard "v" rather than the b/v intermediate sound which is more widespread. That would be good with a pop-up e.g. "this is common in Argentina" or similar. Without it, it is very confusing as I don't think they are being consistent with any one dialect.
This is suddenly becoming an issue once again, or at least I never heard it before. The female voice is pronouncing it with a hard J, and the male voice is not. Given that DuoLingo does not attempt to teach its users about the differences between dialects, a standard really should be set, otherwise learners are going to end up sounding totally bizarre to native speakers.