If I may return to your original question, I agree Bled and Bohinj deserve at least one full day. If you want to visit the Vintgar Gorge, make sure you check it's open before going there. We were hit by a massive ice storm in February and a lot of forests are severely damaged, mainly in the area around Postojna, but Vintgar has also been affected. At the moment it's still closed as the clean up continues, while it's normally already open by early April. I'm not sure when it will open, but it's possible it will still be closed when you're in Slovenia. Their website has the latest information, you can try emailing the caretaker's office if they have any more information on when it might open, you can find their email address here: http://www.vintgar.si/gorge.html
Regarding Vrsic Pass, currently there is still around 25ft of snow in some areas of the pass, it's still closed for traffic and the latest information is it will open by the middle of next week, but obviously that can still change. If it's closed, go over the Predel Pass instead, driving from Kranjska Gora via Tarvisio and Cave del Predil in Italy and over the pass into Bovec. You can check the status of the pass on the website of the Slovenian Traffic Information Centre here: http://promet.si/portal/en/1traffic-conditions.aspx
Regarding your last question, out of the three Piran will be the most similar to Rovinj so it might be worth skipping, then again Predjama Castle is going to be a bit of a detour but would make sense if you combine it with Postojna. In between the Postojna and Skocjan Caves, I personally prefer Skocjan because it's wilder and the underground canyon is simply spectacular, but Postojna is bigger, more touristy and has more intricate rock formations, so it all depends on what you prefer.
I do recommend making a short detour into the Goriska Brda area on your way down from Kobarid, it's a scenic winegrowing region spanning the border with Italy. Then, if you opt for skipping Postojna and Predjama, I recommend driving from the Soca Valley to Skocjan over the Karst plateau instead of taking the highway and stop at the medieval hilltop village of Stanjel along the way. And yes, it's possible to do this in a day and arrive in Rovinj by evening.