I'm no expert but I'm fairly certain a large proportion of the virgin fleet is "leased".
The aircraft are built to the airlines specifications but are owned by a lessor. The ones that are not "FE" I believe were built for other airlines but we're diverted to virgin blue ownership/lease instead.
Don't quote me but I believe there were a bunch of aircraft owned by virgin blue that were bought and then "sold" to internal entities then leased back. If you Google VBNC9 you'll see it's a registered business in QLD and has VUL, VUK, ZPA through ZPK on the register.
VBY & VBZ are registered to VB700.
FE lease linkage :
http://www.flightplanning.com/aus/model.php?q=737-8FE
I'm sure there are people on here with a better understanding of how it all works.