Saladman posted:

How much time do you have for getting from Vienna to Split? What season?

Ljubljana is quaint and cute, not sure it has anything going for it besides a nice general atmosphere. In season, Lake Bled would be the normal go-to day trip from Ljubljana, although both times I've been in the area we didn't visit because it looks beautiful but also pretty much what we can see much closer to where we live. (I will go if we ever had time to hike in the area, but we just would have had time to gawk at a lake and go back.)

Zagreb is not a particularly nice or interesting city. It's fine, but not worth a stop if you don't have to. We did an overnight (±22 hours) a couple years ago which was the perfect amount of time, and I'm someone who vastly prefers 3N first-time-visit stays in any city I go to that has over like 100k people. The train from Zagreb to Split sucks (like 2 departures a day, tiny, no services on board, possibly quite crowded), but it is direct and it will eventually get you there.

Plitvice Lakes are also on the train route from Zagreb to Split, and the train stops somewhere around there -- but I'm not sure how viable it is to get around once there by public transport. The rest of the Croatian highlands that the train passes through is amazingly depressing, just ruined village after ruined village, with the train stopping in dozens of train stations that are in towns with population = 1 person who is over 80 (and five chickens), and the station houses haven't seen so much as a coat of paint since 1988.

Unless you want to reduce time with your in-laws, I would maybe just go directly from Vienna to Split, although you may not be able to do that in one day by train. Alternately, if you can get from Ljubljana to Split in one day, then that would be a good spot for a 2 (or even 3) night stay. Alternately alternately, go direct to Zagreb from Vienna, then rent a car and drive down the coast and make a mini-roadtrip out of it. One way international drop-offs are lol expensive, but getting in Zagreb and dropping off in Split probably won't be too much of a surcharge. With 4 people this would probably be more convenient than taking buses, unless you really just do one single stop and then go direct to Split.

I don't think there are any major pitfalls, besides keeping in mind that there are only like 2 trains a day from Zagreb to Split. English is extremely widely spoken in Croatia and Slovenia.

There's lots of neat stuff around Split like Trogir, and Split itself is also neat for at least 2-3 full days.