The AUS-US route is one of the most popular and difficult to get route for award bookings. However, you have started your efforts at a right time. Seats are only open til Jun 2017 now.
My only advice is be willing to try checking availabilities 11:45-12:15 for 1 night of your convenience - to confirm that’s when they release the following day’s seats. Once you have confirmed that they do indeed open up the following day’s booking at the turn of midnight (AEST), then its a matter of scoping out the days you are willing to fly out of Melb and keep trying the moment these dates are released.
I did the same a few months ago for a New York to Tokyo flight (not as popular, I know). But fortunately I don’t sleep that early anyway.
Book 1 flight at the time. As you are intending to spend 3 weeks there, the return flight would only be released 3 weeks later and there would be next to no chance of you still having available reward seats for the outgoing flight (by the looks of availability now). There is also no discount in booking a return (to my knowledge) on Virgin.
Depending on you Velocity Membership status, you may be able to request them to release some seats too if you are Gold or Platinum (not 100% sure as I am not one myself but have heard bits and pieces about this benefit).
Good luck. Maybe someone with more exact experience and success can elaborate more.