As it's separate tickets I would leave a bit more time than two hours. Even if your flight lands on time, it's processing by Australian Customs that causes big delays resulting in heaps of people missing their onward connecting flights.
Your chances improve somewhat if you're on an Aussie or other passport that's eligible to use Smartgate however when you book the BNE/GLT flight err on the side of caution.
I'm not sure whether SQ will through check to QF or VA if separate tickets and if they don't it means that you'll be up the creek without a paddle if you're delayed in Customs resulting in you missing the cut off time for checkin for either airline which may result in you having to buy another ticket.
On exiting Customs on level 2 at BNE Intl terminal, turn right and you'll see both the VA & QF domestic transfer desks at the end of the terminal. If you're on a QF flight BNE/GLT you need to drop off your bag and checkin at least one hour prior to the scheduled departure time otherwise you'll need to jump on the TBus and head to the domestic terminal with your checked bag and checkin prior to the 30 minute cut off for domestic check-in.
I don't know what VA's policy is with regard to cut off times - someone else of AFF may be able to give you that info but I imagine it would be similar to QF. The Tbus is free of charge to anybody so no dramas regardless of who you're flying with BNE/GLT.