From Vienna to Zagreb, there are several daily bus connection year around, travel time for the 400 km long bus route is about 5 hours, the buses departure either from VIB (Vienna International bus terminal) which is located in Erdbergstrasse or the Stadioncentre which is located in Engerthstrasse. In Zagreb the buses arrive at the main bus terminal, from where you have bus connection to most towns in Croatia. The ticket price for the bus from Vienna to Zagreb, vary depending on bus company, all companies offer a discount if you buy a return ticket, you can see return timetable here.