Yes you are missing something. If you have an email address that is, using your example myname@Virgin.net, then the system is set up so as to stop anyone else, including you, creating myname@virginmedia.com. This has been in place ever since the virginmedia.com email domain was opened up for customer use.
The other option that you have tried just changes the username used to access a particular VM email address. It doesn't change the underlying email address and has to be a third party email address that you already have access to in order to complete the verification process.
So, if you want a secondary email address on the virginmedia.com domain you will need to choose name that differs from your existing primary virgin.net address.
Creating a secondary address will not remove your existing address or any of the emails already in the mailbox.
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GrahamI'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media, I'm a VM customer. There are no guarantees that my advice will work. Learn more
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