Firstly, you are one of those lucky travellers I hear about

, like a friend of mine who seems to always be at the right place at the right time to get upgrade, on the other hand I have never being upgraded.
Now as for being a Velocity Platinum FF, it is too early to tell what upgrades you "might" get with the partner airlines such as Virgin Atlantic, Singapore, Malaysia, Etihad, Delta etc. I have heard from only one couple, the husband was a Velocity Gold FF, who got upgraded from Premium Economy to Business on V Australia on a Brisbane - LAX flight.
All the posters here have already said what I was going to say. Etihad is unlikely to give priority upgrade to Velocity Platinum FF seeing that they don't even give them access to the Business Pearl lounge (from what I read recently in this forum), let alone give you a free upgrade over their Etihad Guest members.
As for Singapore Airlines, you have more chances of winning the lottery than scoring an upgrade, they are known to not give away anything.
Also FYI, the Singapore Airlines/Virgin Australia partnership has certain restrictions. According to AusBT, "
flights to London via Singapore would be available only from Perth, Virgin's current partner Etihad doesn't tap the WA capital but has other capital cities covered, and Virgin wants to avoid treading on the toes of its several codeshare pals.
"Singapore Airlines already flies from Sydney, Melbourne, Brisbane, Adelaide and Perth to Singapore. However, Virgin Australia will not codeshare with Singapore Airlines on cities where it has an existing arrangement with Etihad, such as Brisbane."
I don't know what that means for east coast travellers, i.e whether we can earn points/SC on Melb/Syd/Bris-Singapore-LHR flight booking. Fingers cross we can
You can have a go at cash upgrades, if you call the Virgin Brisbane centre 24hours or less before your flight they might offer you a "reasonable" cash upgrade. I don't know if they have upgraded their system or not, but last time i tried cash upgrade at the airport, they said they couldn't do it and i had to call the Brisbane call centre. But if you call more than 24 hours before your flight (i.e 28 hours), they will quote you the full price upgrade like $3,000.