As of now there are two direct trains Wien(Vienna)-Zagreb per day. Morning train departing Wien-Meidling at 7:03 goes via Hungary and arrives to Zagreb Glavni Kol. (main station) at 12:59. Evening train departing Wien-Meidling at 16:03 goes via Slovenia and arrives to Zagreb Glavni Kol. (main station) at 22:34.
Your cheapest fare option on those routes are discounted international Spar tickets -
http://www.oebb.at/pv/en/Travel_within_Europe/SparSchiene/SparSchiene_Croatia/index.jsp starting from 29 euro. Regular train fare on the route is app. 60 euro
Here you can find details on how to book your tickets Spar tickets -
http://www.oebb.at/pv/en/Travel_within_Europe/SparSchiene/SparSchiene_Croatia/Booking_information.jsp On the route Zagreb-Split you don't really have any other option, then simply to purchase your ticket form regular ticket counter on the station some time before departure. Fare will be around 180 Croatian Kuna