Leaving advice please
Unless you have actually spoken to VM, any discussion about what you might pay for BB only is nothing more than speculation. This is because VM likes to haggle an individual price with each and every customer. For this reason, prices can vary wildly for the same service(s) depending on a whole range of criteria (inculding your own negotiating skills and how likely VM thinks it is you will really leave).
One POV often mentioned on here would actually be to overlap your existing VM service with the new Vodafone service for a short period of time.
When you place your order for Vodafone, you are relying on them installing on time as per their schedule. Nothing to guarantee that Vodafone will do that and you might possibly encounter a delay in installing the new service. In addition, you will have a 14 day cooling off period with Vodafone to try the new service. If you find it doesn't suit you, you could always fall back on your VM connection without interruption if it is still running normally. Doing this though assumes it is practical/achievable from a financial POV.
Depending on what your haggling/negotiating skills are like, you might like to have a read of this past advice from Andrew-G at the bottom of this topic on the business of renegotiating or cancelling
