Road tripping from Venice to Slovenia and Croatia

Not usually
But you have to check with the car agency your are renting from

hey hey Andy
i agree with Christine, ask car rental. do you need some kind of sticker to cross border. also get your IDP (international driving permit) to drive in italy and croatia at AAA for a nominal cost and get it same day. (read at AAA.com)
so many yes and no's here about permit, better to have than not for the $25+. heard from a friend that their friends were asked for one at rental counter, didn't have and were declined the car. that changes the "whole ball of wax" for vacation with last minute changes, with over the moon crowds anywhere & everywhere in europe nowdays, you don't need added stress. seems there is no "off season" anymore and have a feeling october 2023 will be a busy time also. have fun & happy travels
aloha

I see you’ve edited your post so our responses don’t make a lot of sense

Check with the car rental agency- it’s usually fine to take car out of country but they will need to approve

If you rent in Italy—return in Italy or you’ll be charged a HUGE drop off fee

Yes--trip is expanding as we speak. Your input still helpful. Exploring whether it makes sense to get to Croatia or Slovenia by some other means, rent a car there and fly back to LHR from someplace other than Venice and decided given our time frame renting and returning to Venice makes most sense in terms of maximizing our time unless someone else has an idea I haven't considered.

We always rent our cars thru AuroEurope.com or AutoEurope.eu. Check both pricing might be different.

You will have to research your options- can do that on AutoEurope sites- I'd go with whichever was cheaper- that might well be in Croatia or Slovenia as Italy car rentals have been extremely high the past 2 years.

Research your flight options to LHR from Venice or elsewhere.
Flights from Venice- can be a pain to get to the airport if early am- and there is that additional cost

You need a vignette on your car to drive in Slovenia. A car rented in Slovenia would most likely already have that; a car rented in Italy or Croatia probably would not. Google for info on where you would be able to buy the vignette.

Venice makes a good gateway for many to Slovenia/Croatia - I have flown in or out of there a few times to visit the region. But I've never rented a car in Italy. I've always used public transportation or a shuttle service like GoOpti to get between Venice and Ljubljana.

Certainly one option is the car rental you are contemplating at VCE. If that works best for your itinerary and budget, go for it. I usually travel solo, so the economies of scale of public transportation vs. car rental change the higher the number of people traveling together.

But, you could certainly fly into Venice and take a bus or GoOpti immediately to Ljubljana to start - you don't need a car there anyway, though if you do day trips in Slovenia you might find it practical to rent a car while there. And you might not want to drive immediately upon landing in Europe after the long day of flying.

I would work out a itinerary that you want to do and plan the car rental around that.

Thanks Andrew, Christine et. al. Read my recent edit/update--I'd be interested in any thoughts you have.
Many thanks!

I may have missed an entry, but you could do the train/bus connection from Venice to Llubljana. Train to Trieste changing to a bus to Llubljana-both about 1:30 hour trips.