Does it work, is more to the point. My experience of Vodacom4me services is not encouraging. I applied for 500 meg adhoc on top of 1 gig contract and got my contract reduced to 500 meg! Nuff said. v4me should be "vodacom is awful" - never available, very unpredictable accounting outcomes and an e-mail addy that refers you back to 082155!
If you use up your bundle in the middle of the month you can add a bundle to your exsisting contract for that month. The ad hoc bundle expires at the end of the month tho so if u have used your 1 gig and there 2 days left of a month you add a ad hoc 75mb bundle ad if you only use 45mb you forfiet the 30mb. I normally add the 150mb bundle @ R149 as its the best value option of the smaller bundles @ R1 a mb and make sure i use it all. I'm not sure if you can add it to a voice contract but you can to a data contract I have done it a few times.