If none of the impounded planes were to be used in the near future and had been parked there for a long period of time already, then I don’t get how parking vehicles around it would make a difference to VA’s ability or motivation to pay the debt?

Standard operating procedure to reduce potential liability / loss and to stop the administrators from potentially re-positioning planes to another airport to avoid dodging debt. As many of Virgin's aircraft are leased, it means that if the lease was cancelled and the lessor was to request the aircraft to be returned, that the administrators would have to cut a deal with Perth Airport before letting the aircraft leave.