Use your CO record locator on www.virginatlantic.com - it will find the booking (you will need to put in the VS flight number - not codeshare # - and date) and you can select seats, make meal requests, etc. It also shows you the VS locator for future reference.

Do not trust the seat assignment on CO.com for these flights as it does not update correctly in my experience - always do this on the VS flight. There should be no need to call VS, all can be done online.