Welcome to our Community, thanks for coming to us for help with this
Your primary email address, by default, will have a Virgin Media domain (virginmedia.com for recent customers or virgin.net/ntlworld.com/blueyonder.co.uk for legacy customers).
This will be the email address that you use for logging in to My Virgin Media.
If you wish to change this login address to a 3rd party domain, such as your outlook.com address, then please access My Virgin Media> click on the My Profile tab > under My Details click Change next to Username and input your outlook address.
That should sort it out for you.
But, if I've misunderstood your query then please let me know