hkr, you got me confused for a sec
VIPMe is a regular prepaid voice + data service. You can add data packs to it, and you calculated right, for 150 Kuna you can use up to 500Mb of data. Which made me curious because it's much cheaper than my regular VIPMe Broadband (please note that is a pure data service SIM, no voice) tariff where 500Mb cost 200 kuna.
However, you've got a VIPMe SIM and you can use data packs for it but to use VIPMe Broadband service you need to buy another, data SIM, and topup. VIPNet site is not clear about data transfer speeds when using data packs for VIPMe, apart from stating you need to have a GPRS service activated in order to use data packs. Which in other, less marketing-friendly, words could mean litimed choice of protocols where nothing else than GPRS/Edge is available while high speed protocols remain reserved for VIPMe Broadband service. Which honestly adds up, otherwise clients would go for cheaper solution (VIPMe + data packs).
I doubt mailing or calling VIPNet customer service will provide any clear answer but I plan to go down to Croatia next week and could ask some of their tech-savvy advisers to clarify things.