If Vodafone is offering you the opportunity to upgrade, then it will restart the contract period for the deal you go with. Normally the language on these things on Vodafone are pretty clear.
If Vodafone had made me the offer you are quoting, when my Broadband contract came to an end, I'd not be a BT customer now. Worse still, a couple of months after leaving they made me a very nice deal offer, and then a very odd one which I imagined I'd dreamt until someone else posted about that very same odd offer!
