In order to link your Booking.com account to your Virgin Red account the email address used for both accounts must be the same.
This is because when you ask to link accounts, Booking.com searches its database for the email address used on your Virgin Red account, and if it can't find one it opens a new Booking.com account for you.
If you want to change your email address on your Virgin Red account you can do this yourself either via the app or web. Go to the account page and click the little "edit" above the email address. If you need any help please contact Virgin Red Member Support