You can use VIP prepaid card only for internet/data (not voice), price is 20kn and you have 20kn on this card (1Mb=1kn). You must ask for VIPme Broadband card. This is details on VIP page: http://www.vipnet.hr/cw/show?idc=8749583&idnews=639306

You don't need to buy USB modem if you have one or you can use you mobile phone for connection (voice services not working) but you must configure internet connection mobile phone APN:data.vip.hr