We are planning to rent a car in Milan and travel to Croatia then back to Milan and return the car there. Should we expect any difficulty or issues doing this? Do we need a "green card" and where to get this and can we get ahead and expected costs? I've also heard we need something called a vingette to travel through Solvenia is this true and again costs and can we get ahead?
Thanks,
You do need to notify the rental agency of your intention to cross borders, so I would inquire with them about the vignette. I think you just purchase it when you enter the country, but why not just see if the agency can assist you? We pay them enough, they should!
https://www.slovenia.info/en/plan-your-trip/getting-to-and-around-slovenia/to-slovenia-by-car
Certainly inform them where you intend to go. Italy, Slovenia, and Croatia now all being both EU and Schengen, and you can now get to Southern Croatia without taking a ferry or going through Bosnia Herzegovina, you may not have any issues. If you plan to leave the Schengen/EU area to Bosnia, Montenegro, or other countries, that may be an issue.
In addition to letting them know the countries you will be in, if you plan to take the car on a ferry, inquire about that. Some places it is not even an issue, many places it nullifies any insurance and violates the rental agreement.