I had the same problem with my MTN Huawei card. Just inserted the VC prepaid sim, and it connected no problem.... BUT... only 3G speeds.
Apparently, since it's not a VC sourced datacard, they don't have record of you being an HSDPA user, so you're only provisioned for 3G.
Vodacom3G helped getting the provision to to HSDPA done, since I'm using a prepaid sim the cost is the same for 3G and HSDPA.