Gave them a call and they said you can buy and register now, but must keep the account alive for 35 days from when you activate to be eligible.
"The monies will be credited to your nominated bank account via EFT or PayPal within six to eight weeks."
"Once your 3GB starter voucher expires in 30 days you must recharge or sign up to a subscription plan to remain active for at least 35 days."
This would mean you have to sign up for at least another $19 / 2GB / month-to-month plan, making it more like $68 for 5GB over 60 days I guess?
Must purchase instore, from Dick Smith Electronics, Harvey Norman, Melbourne University Bookshop, UNSW Bookshop, All Phones, Rick Hart, IT Works, Portacom, Titan Interactive and The Good Guys retail outlets only.
Device info here with some details of 3GB starter data voucher:
http://www.vividwireless.com.au/devices-plans/devices/vivifi
Can connect up to five devices.
It's free on a 12 month plan.
If you're in a coverage area and want fast internets while roaming, vivid look pretty good. Didn't notice they've moved into more cities than Perth now. :)
$79 unlimited plan seems interesting too.