respect hkr,
seems to me you discovered a potential loophole in pricing policy of VIPNet
that is allowing for cheaper broadband internet on a simple prepaid voice SIM card armed with data packs than on a dedicated usb data card+data SIM combo...strange but true, congrats! Mind the APN for VIPMe Broadband, that is data.vip.hr.
Keep us informed however of actual speeds and VIPNet coverage if you're about to go for some weeks to Croatia this summer.