In theory, VM's own policy (and clearly stated in the Ts & Cs which customers agree to when signing up) is that any email addresses are closed down and all of the email within the mailbox are deleted 90 days after leaving. This is what should have happened to your ntlworld address a few months after you originally left VM to go to BT. However as you have found VM can, for various reasons be somewhat lax in actually doing this and some mailboxes continue to be 'live' for years sometimes decades after and the ex-customer just carries on using them in blissful ignorance of their somewhat parlous state.
Even though you are a VM broadband customer now, the email address would have been associated with the original account broadband account which would now have been long since deleted, the mailbox and address are regarded as 'orphaned' and as such are liable for deletion at any point. Every now and again VM carry out a periodic sweep and cleanup of orphaned mailboxes and it is likely that this time, your's by chance was one that got caught and disabled.
