The Qantas 737's are not Boeing orders, they are unwanted American Airlines aircraft leased to QF for a few years. QF has not published what will come after them when they are returned at the end of the lease period??????
Virgin is a big enough company that can get their way with Boeing if required. I don't think (but I could be wrong) available slots is a factor. I think it is more likely to be hard bargaining in a buyers market.
Have a nice day