After extensive support tickets to Vodafone on this subject, they confirmed that after the mandatory 24 months subscription the device is left in an unlocked state for you to use as you wish, all your devices will continue to work as will the hub, all that you lose is the V-Home monitoring and cloud storage. They told me that even if you cancel the subscription after 1 month you will still be billed the full 24 months in one go. You cannot add a 2nd hub as a wi-fi mesh extender if it is keyed as a Main Hub, they apparently do sell extender Hubs which are keyed as Subs but which you have to pay full price for from day one at £99. I bought 4 Main Hubs and managed to set one up as a Main Hub and one other as a Sub - the other 2 failed to configure as anything other than Main Hubs.
Also, Plume confirmed to me on a support ticket that Vodafone have NOT bought Plume licenses for these devices and I can confirm that you cannot configure Plume on them.