I would definitely not rent a car from Vienna - that's going to incur a huge one-way drop fee later. Either take the train from Vienna to Ljubljana or fly. (Or to save money, maybe FlixBus, which might be slightly faster than the train.)
I loved Ljubljana. Personally, I would spend all three nights there and day trip to Bled. It's easy to get to Bled by bus from Ljubljana. You could start the day by taking a train to the little town of Radovljica near Bled, explore the charming little town (and perhaps its quaint little bee keeping museum), then take a bus on to Bled, then bus back to Ljubljana later. But I would consider renting a car just for the day instead. (International Driving Permit may be required, get one ahead of time before leaving for Europe.) Drive to the town of Skofja Loka (lovely little town) then from there do the scenic drive through the mountains, via Jamnik and Kropa (not via the fast freeway via Kranj) and down into Bled. You can likely return the car after hours by dropping the keys somewhere. That's what I did last time when I rented from Avantcar in downtown Ljubljana.
Zagreb has its fans, but I wasn't one of them. I would probably spend just one night or even half a day. and rent the car and drive to Plitvice the same evening, get up early and enter the park when it opens at 7am to see it before it gets crowded, leave by about 3PM (should be plenty of time to see the park) and on to Rovinj late. You could even skip Zagreb altogether; take a train instead from Ljubljana to Rijeka and rent a car there and drive to Rovinj - or take shuttle service like GoOpti directly to Rovinj and rent the car there. From there on to Plitvice and head south as planned, return the car in Dubrovnik.
There are a lot of choices for your nights after Plitvice. Many would be tempted by an island like Korcula or Hvar. (You could ditch the car in Split and take catamarans and buses the rest of the way to Dubrovnik with a stop on an island.) Really up to you if you prefer something inland like Mostar or a trip down to Montenegro. Do some more research, watch Rick Steves videos on YouTube. With your limited time, you'll have to make some choices and not see everything.