I had a look through the 2022-23 annual report and if I'm interpreting it right (high chance I'm not), the highest relevant costs are indeed staffing, but that's followed quite closely by vehicle maintenance / repairs. There's obviously not much that can be done about staff costs, but smaller trains = less maintenance, less surface area than can be graffitied/vandalised, etc. Vlines fuel costs aren't as high as I was expecting, although if a new generation of sprinters was ordered I imagine they'd have more efficient engines and be at about the same fuel efficiency as vlocities (my assumption is they'd do worse at the moment due to having an engine for every car and being older)

But really anything is doable if the political will is there

(Edit: this entire comment is just an assumption, to be clear. I don't think there's any more specific info out there, so the only thing I could think of was seeing whether or not fleet costs outweigh staffing costs annually or not, and they currently don't, but some staff included in the annual costs wouldn't be relevant to a shuttle service so that'd being the actual cost down a bit too. My final assumption is that staff costs and fleet costs work out about the same annually)