When you switch to Vodafone, you can either transfer your number (hopefully, they do seem to muck it up sometimes) or get a new one. Vodafone don't offer a broadband only service.
So no you will not be contracted to B.T. for anything.
The phone line will be delivered digitally and will be connected via the router.
If you are in a Full Fibre are it will delivered via that, if you're not in a FF area, it will plug into the BT/Openreach master socket.
