So you reckon they are trying to charge me full fare? and telling me thee are no SparSchiene tickets.
Yes, they either misunderstood you or specifically try to sell you full fare tickets instead of discounted ones. So, our advice:
1) Email them once more and try to persuade to sell discounted tickets (give them link on their own website about Spar tickets etc.)
2) If you fail with step one (what will be a pity and disappointment with Austrian Railways service), then simply trick them back by purchasing online ticket from Vienna till last Slovenian station before Croatian border (Sevnica). Those tickets can be easily bought online directly and printed on your own (
https://ticket.oebb.at/bin/frame_ticket.pl?ticket=jticket.pl&LANG=EN). Since Zagreb is located almost by Slovenian border, then ticket from Sevnica to Zagreb won't cost a lot - full fare is around 8 euro, so in total you will pay 29+8 and not 51 euro. You can buy Sevnica-Zagreb ticket in Vienna, so you won't need to get off the train there and be able to use direct Vienna-Zagreb train with two tickets for separate parts of the route.