I would just book a fly drive and not take the car as VA prices are about the same as VH surprisingly. They are usually a bit cheaper as VH charge DD adult price. It is typical, Disney world.com have great room rates and Virgin flights are more expensive than usual.
Just looked at my card and it says 20% your next holiday-book between now and the 26th july-the small print doesnt mention about not including Flydrive.However it doesnt cover attraction tickets,fuel surcharges and ADP.Or cruise holidays accomodation only or car hire.
Regards the tickets they must give discount then add excursions on What happens if free dining comes out but already booked without.Will they add it free of charge