Driving from Venice to Istria (and back)

I haven't driven in Europe, but I crossed the Slovenia/Croatia border on a bus a few years ago and noticed that there was a back-up going in the other direction. Just want to warn you that there can be substantial traffic (beach-goers, I'm guessing) passing through that Schengen/non-Schengen border, so leave some slack in your schedule to allow for possible delays. It's probably worse around holidays and weekends.

I've driven in both Slovenia and Croatia but not crossed the border between them on my own wheels. My Slovenian car already came with a vignette from the Slovenian rental car company, but an Italian car surely won't have one - you will need to buy a vignette for sure at the Slovenian border (gas station on either side of the border?) to drive on Slovenian highways.

I paid highway tolls in Croatia with both my credit card and with cash (Kuna). You can try your credit card, but I wouldn't guarantee it will always work. So I'd probably hit an ATM as soon as you cross that border. You'll probably want some Kuna anyway, right? I used my American chip and signature card (I had a chip and PIN card too but didn't need it). I don't remember if I ever had trouble with a card, but I drove a long ways in Croatia so paid quite a few tolls.

Are you sure you'll be able to drive an Italian rental van into Croatia?

You'll need an International Driving Permit (IDP) along with your home driver's license to drive in Slovenia (and probably in Croatia and Italy).

You can use your smart phone (or maybe your tablet) as a GPS - Google Maps works great, even if you don't have mobile phone service. Just download the maps ahead of time in "offline" mode.

Definitely get that IDP. It has been posted on the forum that the fine for driving without one in Italy is over €200. Although I'm sure you don't intend to do anything to attract the attention of the police, some other driver might bump into you.

If possible, avoid border crossing at the weekend, it is always very busy!

I prefer to pay Italian tolls with cash euros and go in the line that's staffed - look for the sign of a hand holding cash instead of just cash by itself. The price to pay is displayed and the cashier makes change. I feel it's the simplest procedure, reducing the risk of possible malfunctions, misinterpretations, or bills/change acceptance issues with an automat (not that I've experienced any problems recently).

The car rental agency will enquire at the time of booking into which countries you will drive.