Your question is a bit like, "Can I pick up my Telkom phone and connect to some infrastructure provider other than the one I have contracted to for the service and thus avoid paying my contracted service providers rates"
Of course, if one wants to stretch a point, you can connect to an "ISP" (I wonder what you regard Vodacom as in this instance - since what they are being here is an ISP) over 3G and get more than 1G for less money per unit data (I am guessing this is what you meant) - for instance Sentech's service could be thought of as 3G since, AFAIK, they are also using UMTS as a bearer and that is considered 3G technology, although it is not marketed as such. However, this would require a Sentech contract and not a Vodacom contract.
There is also the possibility to connect to an APN other than the standard one provided by Vodacom (internet.GPRS) however you would still be accessing it across their network, and that is what you are actually being billed for, network usage. The access point used is not really the issue when it comes to being billed (although it is technically possible that this could be used as a factor when it comes to rating your usage)
