Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,182
Expedia is the only place to upgrade your ticket, except on the day when you can check with Virgin staff at the airport.
Because you seem to have been using the wrong terminology, Expedia probably assumed that you wanted to buy a Comfort + seat, which you do directly with Delta. In their defense, in the US they probably book 50 or 100 Delta tickets for every VS ticket...
Call Expedia and ask how much an upgrade to Premium Economy would cost.