Terminology can get a bit confusing when exchanging messages on here as VM has made the system unreasonably and unnecessarily complicated through multiple recent changes.
As a VM customer you will have a 'primary' login to 'My Virgin Media'. This will be the 'My VM' login where you can manage your services, see bills, view your package details etc. etc.
Via that primary My VM login, you should also be able to see all the email addresses that are currently associated with your VM broadband service.
This link below shows how you can check your other associated mailboxes when logged in under the primary My VM account
Virgin.net access for non virgin customers | Virgin Media Community - 5593360
You should be able to log into the primary My VM via the 'Sign In' button here
https://www.virginmedia.com/myvmo2
In the past, you used to be able to manage secondary email addresses through the primary My VM but VM has changed that so that each mailbox now has its own My VM login to make changes (such as password resets etc.)
VM is also prompting customers to link a third party email address to each mailbox in the name of increasing security. This is adding a further layer of confusion. When this happens, VM requires each VM mailbox to have its own unique third party email address to run alongside it.
I think (from what you have already written) it sounds like you can access the primary 'My VM' login but your BY addresses are missing from that. If that is the case, your BY addresses may have been 'orphaned' from your VM broadband account (which can happen if you leave VM and return later on and for other reasons).
VM has been closing down mailboxes that are not associated with an active VM broadband service.
To add some further VM 'confusion' there seem to be a number of topics today mentioning problems logging in to My VM so there may be some tech problems going on as well just now.
I think you will need the VM forum team to look at your account/mailboxes and advise what is going on. Someone from the VM forum team should get to your topic and reply, uusually within a few days of your first post.