you should be able to buy stuff with VC offline but i know after every purchase i make it checks with the 2K servers so i hope they weren't stupid enough to mess up the offline integration. i haven't purchased anything but the game and have earned all my v but still it should be avaiable offline as well
i disabled my wireless adapter and you're right! this is dumb you SHOULD be able to spend VC offline but it looks like right now you can't. is it at least letting you earn VC in other modes or by doing th tutorial? it might just be an early glitch...a bad one though.