Vignette and International Green Card for Slovenia, Croatia and Bosnia.

Re this part of your question:

I spoke to my current auto insurance folks and they said I am covered for cars rented only in the 50 US states and territories. So this means, I would need to purchase insurance for renting in Slovenia, Croatia and Bosnia, correct? If yes, how do I go about doing this? Or will my credit card kick in as the primary (and only) insurance provider (when I decline CDW) for the rental car in the absence of my US auto insurance? But the problem is that I am renting through Avis and they are not giving me the option to decline CDW...at least not when I book online. Haven't tried speaking to them.

This is a complicated question. You won't get a definitive answer, here, to this money question.

Start your research with Rick's advice,

go here for that

Then go over to the applicable Tripadvisor Travel Forum (applicable for your locations) and ask the question there.

As you already realize, this is a money question, potentially a big money question to you, and you will want to nail it down with specifics you can count on, not opinions from people making guesses.

What has been said before is: If in the end you're in doubt, pay the extra for the CDW. Or, bet the market value of the rental car. One or the other.

If you rent the car in Slovenia, it will probably have the vignette sticker already attached. If not, you can probably purchase it from the rental agency.

On a flow chart I have showing the driving particulars in each country, it indicates that there is a fee for highway driving in Croatia (none shown for Bosnia). It does not state if the fee comes in the form of tolls or a vignette sticker. Considering that Croatia and Slovenia were once part of the same country, and I know for a fact that some other parts of ex-Yugoslavia do not have tolls, I would expect you need a sticker.

EDIT: And I see you have double-posted, and I am offering some contradictory information to the other person that replied. Well, I'm pretty sure the vignette sticker is something you can figure out once you arrive at the rental agency. Can't help with the Green Card, though.

Where do you plan on renting the car? I've rented in all three countries when working in the former Yugoslavia but didn't cross any borders with them. All 'in country' requirements were taken care of by the rental agency.

Slovenia definitely has toll roads (very new/modern)and I paid with coin going through the booths.

You won't find a toll road in Bosnia (not many guard rails either) and I don't recall seeing any in Croatia.

You'll want to check with Avis (and I'd do it over the phone not online) that it's OK to rent a car in Slovenia, for example, and take it into BiH.

What you should be aware of is that driving in BiH is not like driving in Virginia; or Slovenia/Croatia either. Assume the driver of the car coming at you has a death wish and that his vehicle probably hasn't had a safety check for awhile; if ever.

WRT insurance I didn't rely on my policy at home nor my credit card; paid the CDW to avoid heartburn in the event of a scratch or dent.