You can buy VC if you go to your players stats and increase one to where it costs more VC then you have.  For example, if you have 100 VC, go the lay up stat and increase it to where it will cost you more then 100 VC and a prompt will come up asking you if you want to buy more VC and then it will send you to the store.  At least that worked on PS3 and I bought 10,000 VC for $2.99, hope this helps.