Venezia 0920 - 1250 Villach
ICB, compulsory reservation 7€
Villach 1452 - 1856 Zagreb (there would be a train leaving Villach at 1253 - connection can't be guaranteed)
Zagreb 2133 - 0549 Split
Zagreb 2250 - 0656 Split
That seems to be the most reasonable connection; however there would also be the possiblity to take the EN from Venezia to Zagreb and continue with an ICN from there to Split, but that would mean an arrival in Zagreb at 0418.