Vodacom 10Gig Top Up Contract

I took out a 24 mths 10gig data contract with Vodacom during Dec 2015 because I thought this will save me money. Up to this point I used prepaid data. I bought 5 gigs at a time it is more expensive than what the contracts offered me (10 gigs for R499). The contract was activated on 28 Dec 2015 and prorate data (+- 1,5 gigs) was given to me (billed at the end of Dec) with a recharge of 10 gigs on the 1 Jan 2016. Big was my surprise to see that 10 gigs of data were depleted within 10 days. How is this possible? Previous months I would use my prepaid data (5 gigs) for almost 20 days at a time. Where did that data go to? It is like a bath full of water and you pull the plug! Vodacom (082155) only tells me it is internet. No more detail can be provided. I would like to cancel this contract because to me it looks like the more expensive offer. Now Vodacom tells me I have to pay R8000 to have it cancelled because it is more than 7 days since activation. Surely within the first month if you are not satisfied with the product you can have it cancelled? It is however liking hitting a wall and not being able to speak to someone who is willing to assist?