Split and the islands for 4 nights

How much time Split deserves depends on personal preferences. Some people love Split; some of us don't. I didn't care for it that much. Two nights was more than enough for me - one would have been enough for me, but I think at least visiting Diocletian's Palace is well worthwhile. Read other opinions on Split to try to figure out whether you will like it or not. I liked nearby Trogir a little better - more charming and intimate than Split, though Split is probably more interesting and has more to see.

I have not visited Hvar, but an obvious alternative to it is Korcula. There is daily catamaran service between Split, Hvar, and Korcula Town (or if you have a car, there's a car ferry from Split to Banja Luka at the other end of Korcula.). There may be a catamaran from Korcula Town to Dubrovnik, but there is a daily bus too. (Uses a short ferry at Orebic which you can use with a car too to get down to Dubrovnik.)

You may or may not need a three night minimum stay at lodgings on Korcula in September - that may be only in the summer.

Are you prepared for a steep one-way drop charge for renting a car in Slovenia and dropping it in Croatia (Split)?

Five nights in Dubrovnik is a long time, especially given your relatively fast itinerary get down there. Dubrovnik is lovely but also pretty small and explored quickly. It gets mobbed with cruise ship tourists during the day, so it's a decent base for day trips. The old town is probably not a great base just for chilling and relaxing due to the crowds. You might consider basing nearby in Lapad or Cavtat if you wish to relax there.

But if you wanted to add more time before Split, you might consider stopping in Zadar, Sibenik, Krka National Park, an extra day in Istria, or even adding an extra day in Slovenia.

Here's one option: spend one night in Split, during which you will buy your ticket for Vis, and find the nearby bus station and check schedules for Dubrovnik on the 23rd.

Next morning, take the ferry to Vis and stay 3 nights. Stay in (or at least visit) the gorgeous little fishing visit of Komiza. From there, on a sunny day, you can visit the blue cave at Bisevo.

On the 23rd, take the early morning (around 5 am) ferry back to Split, hurry across the street and catch the 8 am bus for Dubrovnik, which will get you there around noon. There are several buses during the day if you miss this one. It's a pleasant drive.

Vis is a lovely, small island. You might want to rent a car for a day to explore. We were there in September & the weather was gorgeous.

The middle part of your trip ie Rovinj to Split is very rushed, then IMO you have 2 -3 days too long in Dubrovnik.

Check one way car drop fees, as this maybe prohibitively expensive.

Two nights in Rovinj only means one day there, which is short changing the region. Split and Trogir each deserve a day of your time.

Although Vis is pretty, it doesn't link well with getting to Dubrovnik, which is half a day away, so I would opt for Hvar (stay in Stari Grad not Hvar town) or Korcula. By the third week in September, the weather maybe ok or Autumn may have set in ie cooler and wetter. I would therefore go to somewhere other than Vis.

Thank you all for the replies. Our Dubrovnik stay will likely include a day trip to Mostar so we figure the extra time there will be good for us. We'll consider adding another night in Rovinj but then would need to decide whether that comes from Split/Island time or Dubrovnik.