In the World War II most Australian army personnel were allocated a service number which denoted their state of origin; N for NSW, V for Victoria, etcetera. Enlistments relating to the Second AIF were followed with an X. Also during World War II, Old Melbourne Gaol was drafted into service to hold military prisoners, most of whom had gone absent without leave.
Given the "VX" prefix on this scratching, I think it's safe to say that this cell once held a Victorian member of the AIF who was doing a bit of time for absenteeism.
These days of course he'd probably just update his status on Facebook rather than scratching his service number into the wall.