Help determining transportation: Venice to Ljubljana to Rovinj to Split

It depends on date, but if you are flexible on the order, I would check Venezia Lines for ferries from Venice to Rovinj and Pula. Get car there.
Googel Maps and/or RometoRio should bring up the public transport options.

Best and cost effective will depend on the size of group, how much time you would spend in Slovenia?

I agree with the suggestion to take a ferry to Rovinj and rent car there. If that doesn't work for some reason I'd take public transportation to Croatia. You can always check on the cost of picking up in Slovenia and returning in Croatia but I suspect it will come with a hefty fee. I once took a train between Venice and Triest, Italy and then a taxi to Piran (less than an hour, was only €50, but that was a few years ago). From there easy bus over to Croatia.

I will be traveling the same route (in reserve), starting in Split ending in Slovenia. I found that Sixt rental car one-way drop fee to be the best and most efficient option for us, so we (two couples) are renting a car in Split and returning it in Ljubljana and then taking the GoOpti shuttle to Venice. The one-way drop fee was only about 100 euros (9 day rental period) and GoOpti shuttle from Ljubljana hotel to Venice is 20 euros per person. Just a suggestion...

Thanks for the replies thus far.

There are 3, maybe 4 or us, scheduled into Venice May 20, then Ljubljana May 24, then Rovinj on 27th, then Zadar the 30th, then Split June 2; leaving back to USA on June 5. The plan has been for these cities to be bases from which we could do excursions. I already have accommodations arranged in all the cities, and feel set with the itinerary.

These seem to be the options:

Take a bus from Venice to Ljubljana, then rent a car to see Slovenia.
Then take a bus to Rovinj, rent a car for Croatia, turning the car in at the Split airport.

Or, take a bus or ferry to the city in Croatia closest to Slovenia, rent a car there, and keep it until we depart from Split.

Other options or suggestions? What companies and best way to book?

From Venice to Rovinj the ferry choices seem to be Atlas Kompas or Venezia lines. It looks like about E165 for 3 adults one way on Venezia. I was nearly ready to book a ferry line to book for early September. Based on the comments on Tripadvisor for both ferry lines, though, I'm becoming discouraged about the ferry. It seems great when the ferry is running smoothly but potentially miserable otherwise. The ferries seem to break down several times each season, and can't run in larger waves because they are not large vessels (e.g. not car ferries). Then the ferry company (based on comments) may provide an uncomfortable bus (no air conditioning, no bathroom, slow), according to several complaints. I'm beginning to think GoOpti or similar would be a better choice for us as we don't have a lot of leeway in that part of our schedule. I know you can't believe everything on Tripadvisor but there seems to be a running thread of occasional service disruption issues that aren't handled well. I'll follow any additional comments here with interest.

I can't help you get from Venice to Ljubljana, but we rented a car there and drove around Slovenia for a couple days, returned that car, then took the train to Rijeka and rented a car for 10 days to do all of our driving around - Rovinj, Plitvice, Split, Dubrovnik, Mostar and Kotor. It's a fairly easy drive from Rijeka to Rovinj. So we rented and returned that car in Croatia and didn't have to pay any excessive fees.

Tigerfan, it sound like you're doing about what we're planning to do - thanks for your input.

Is there a particular reason why you chose Rijeka to rent a car from? From whom did you rent. We'll be returning the car in Split.

I chose Rijeka because the train from Ljubljana goes to Rijeka and I preferred train to bus. We rented from Oryx at the harbor which was walkable for us from the train station.

I have taken the Venezia lines ferry Venice to Rovinj many times with no problem. The upper VIP lounge is more spacious and worth the extra few Euros. Depending on the day, it goes via Pula or Porec. This route is hassle free and charming.

Car rental options at Pula airport or downtown are more extensive than in Rovinj itself. Not parking a car in Rovinj is helpful.

You may want to check out bus options Venice to Ljubljana and then on to Rovinj - e,g. on the Croatiabus website. Bus is also less stress than navigating a border area in a rental car. Some of the dots are not connected yet between Slovenia and Croatia roads.

We also opted to stay in Rovinj and then drive out to visit the hill towns, etc... It's true that you will most likely be parking your car outside of the historic part of Rovinj in the big lots for that purpose. We were able to drive up to near our hotel to drop off our backpacks (could have just carried them, but that's what was suggested to us) and then when we checked out, they gave us a ride out on a golf cart (also not necessary, but very sweet). In between, to use the car to visit the hill towns, it's an easy and pleasant walk past the market to get to the parking lots.