It will be interesting to see how it plays out.
If you look at
www.united.com and make a dummy booking Virgin Blue is listed as a Frequent Flyer partner. I know previously they had a limited code share relationship on certain Aussie routes, but this look as if Velocity members will also be able to earn on all UA services.... i will presume it will be reciprocal. Looks like someone updated the website before the announcement!
I think V will take traffic from QF and even NZ. Actually I think NZ have the most to lose as usually they have competed on price from this market (ie always been cheapest).... now that trophy will go to V I guess, so NZ's bookings will suffer.
If VA/UA decide to codeshare on the route (or UA pull out in favour of codeshare) it could change the game somewhat.