Van stock is the parts and materials carried on the vehicle rather than fetched.
What it buys
Completion on the first visit. A job finished on arrival costs one trip; a job needing a part costs two, and the second is unbilled — see reducing callbacks.
What it costs
Cash sitting on a vehicle. Multiplied across a fleet it is a substantial amount of working capital in a form that earns nothing until used.
It is also the stock most likely to go missing or expire, because it is the least counted.
Setting the list from history
Which parts actually get used, and how often, taken from completed jobs rather than from judgement.
The pattern is usually stark: a small number of parts cover most jobs, and a long tail is carried "just in case" and rarely moves — see dead stock.
Stock the first group in depth. Order the tail as needed.
Replenishment
A van restocked when someone notices runs out. The reliable arrangements are a set list checked on a schedule, or restocking against what each job consumed.
Where a runner collects parts, that time is a real cost and is worth measuring — frequently it is cheaper to carry more stock than to keep making the trip.
Counting it
At least periodically, per vehicle. Van stock is usually the least accurate part of an inventory figure, and it is what makes inventory turns unreliable if ignored.