Airbus (and Boeing to some extent) overbook their sales book. So - with VA's airplanes orders being cancelled and delayed, they may not be completely available to others.
For example - if there are 100 orders scheduled for a year, Airbus may be expecting a certain percentage to cancel... so they might only be capable of producing 90 a year.. so some of Virgin American's orders might have only existed in thin air... (and thus other airlines might not be able to pick all of them up)