Are there any problems renting a car in Vienna and driving through Slovinia?
No problem, but in case you plan to drop off the car in Slovenia instead of returning it to Vienna, be prepared for a hefty surcharge.
You mean other than making sure the rental company is aware of and approves such use?
Just keep in mind that you need to buy a vignette to drive on the motorways in Slovenia!
A vignette can be purchased at the Slovenian border. It is required on highways. You must tell your car rental company that you plan to drive in Slovenia and an additional charge is likely. But driving a rental car in Slovenia is not a problem as the highways are excellent.
In April we took GoOpti from Vienna (about $20.00) to Ljubljana then rented the car there for 7 days.
Thanks for the info!