Welcome to Vodafone Community
This Vodafone help link should explain > https://support.vodafone.co.uk/Account-bill-and-to
You'll receive this in approx 2-4 weeks and taken via direct debit.
Then Vodafone will send a £0 bill and sorry your leaving letter which confirms account cancellation.
Please consider not cancelling the Direct Debit until Vodafone have taken any final payment(s) and if applicable applied any Refunds. And you have received the final bill by post, paid by Direct Debit and a £0 bill and Sorry Your Leaving Letter has been received that confirms the account is closed. (Please retain this paperwork)