You won’t be able to manually connect to the Vodafone network. Three/Vodafone are using MOCN, or Multi-Operator Core Network, which is a network-sharing technology defined by 3GPP that allows multiple mobile network operators (MNOs) to share a single Radio Access Network (RAN) infrastructure. Your device will automatically connect to whatever mast has the strongest signal, be that a Three mast or a Vodafone mast. MOCN is being enabled on a mast-by-mast basis and will take years to complete.