vienna to bled by rental car

Yes, it's going to cost a lot to do a one-way rental like that. Plus, you'll need to buy a vignette driving sticker for driving on Slovenian highways. Buy one at the border or risk a steep fine. (A Slovenian rental car will come with one included.)

I would stick to your original plan. Yes, it's a six hour train ride - but it's at least a four hour car ride. Personally, I'd rather be able to relax on the train and take in the beautiful scenery through the alps from the train window.

But if you really want to save time, there are direct, affordable flights from Vienna to Ljubljana. If Bled not Ljubljana is your real destination, you can rent a car at Ljubljana airport and drive right to Bled - the airport is about as far from Bled as it is from Ljubljana. Don't skip Ljubljana, though - it's a lovely, charming town.

Is renting a car in Vienna and driving into Slovenia a hassle or very expensive to drop off in ljublana?

Probably. Have you gotten a quote?
They cannot re-rent a car in Slovenia that has Austrian license plates, so it has to be transported back to Austria. Who do you think pays for that?

I have rented cars in Florida with Texas plates, and cars in Texas with Florida plates, but it does not work that way in Europe.

Oh, and if you do drive an Austrian rental car to Slovenia, be sure to stop at the border and purchase and attach a Slovenian vignette.

Ok those were the answers I was expecting. I will stick to original plan and get myself to ljublana either by train or goopti, spend night there, then rent car next day to freely tour bled and alp area for another 2 nights then drop car off in ljublana and train it Zagreb. Thanks!

Consider the scenic detour between the town of Skofja Loka and Bled when driving from Ljubljana to Bled. The town of Skofja Loka is lovely and worth a stop, and the drive from there through mountains - via Jamnik and Kropa, not via Kranj and the freeway - is beautiful with some breathtaking scenery down. The road is a little narrow through the mountains but it's not a difficult drive if you take it easy.

Thanks Andrew! you've been a great help with planning this trip. We were going to do just day trip to bled by train but I think it would be nicer take 2 days to drive and have time to stop and hike etc. Thanks for suggestions!

You're welcome! I hope you enjoy your trip. I love Slovenia!