There are no international trains (in the moment) between Greece and Bulgaria. You need the bus. 

From Vienna to Bulgaria you can get via Romania, with the Night train from Vienna or day train. 

Via Serbia no international trains in the moment.

In short: forget the train for this trip by this way, unless you are a 123% railfan. The route that is possible is so much longer and needs so much changes and bad connections, that its only for diehards. OR if you wanted to visit these countries also anyway- RO, BG.

You can also go via Italy-east coast then ferry-other as site suggests, there is as for now NO free ferry (there are many, from various ports), but most give discounts-also for ju/senior etc.