Vehicle costs are everything the business spends to keep vehicles on the road: fuel, maintenance and repairs, insurance, lease or finance payments, tax and registration.
In a mobile trade this is usually the largest cost after people.
Fixed and variable, mixed together
The split matters because it decides what a quiet month costs.
Fixed — lease or finance payments, insurance, registration. These continue whether the van moves or not.
Variable — fuel, tyres, and wear-driven maintenance. These fall when work falls.
A fleet financed on long leases is a heavy fixed cost. The same fleet owned outright is mostly variable, at the price of the capital tied up. See fixed and variable costs.
The cost that never reaches an invoice
Drive time is paid labour that appears on no job. It is frequently larger than the fuel.
Which makes routing a bigger lever on vehicle economics than fuel price ever is — see service zones and the trip charges that make distant work carry itself.
Purchase versus lease
Not a question with a general answer, but the trade-off is consistent: buying ties up working capital and produces an asset that depreciates; leasing preserves cash and raises fixed costs.
A business short of working capital and growing usually leases. One with cash and stable demand usually buys.
Recording it properly
Fuel used on a specific job can be costed to that job; general running cost is overhead. Most businesses treat all of it as overhead, which is acceptable and slightly understates job costs on long-distance work.
Personal use of a business vehicle has tax consequences in most regions and is a question for your accountant rather than a judgement to make yourself.