I gave my VS to the daughter not that long ago when I bought a new Caprice, whatever, but the daughter just came back from Brisbane where she got around 650kms / tank of 58 lit. that's Like Yeppoon to the Gold Coast.
Nothing special on the VS but for a good CAI setup, all the simple stuff was done 6 months ago, plugs - leads - fuel filter - air filter - cleaned T/B, not too bad for an engine with 250k on it.
From what I see the general fuel use per tank for a car in a good state of tune is 450 +/- 25, that useage is near doubled with a heavy foot, like racing the traffic lights etc, hard acceleration / hard stopping or excessive use of the power button that allows more fuel in.
If the car is still getting lousy mpg one might be checking all the basics, then revising driving habits, silly things like a tight throttle cable will make ya push harder than needed on the throttle, the opposite that ya might think, with a light throttle it's easier to feel exactly how much your pushing it.