I flew Adria FRA-VIE a month ago and I was very positively surprised, excellent inflight service, friendly crew and nice cabin.
A nice change from OS and LH.

Adria flies a triangle route as Kiwi Flyer mentioned, i.e. LJU-FRA-VIE-LJU and LJU-VIE-FRA-LJU.
Although passengers travelling from FRA to LJU are advised to take a nonstop flight, flying via VIE would mean a normal transfer as FRA-VIE and VIE-LJU are regarded as two different flights.