I would imagine that your contract of employment had a section written in it regarding the general conditions of using the company vehicle.
This would normally state that it is your responsibility to keep the van in a reasonable condition however, I would suggest that you read through your contract as I have never seen anything written that states an employee would be liable for any repairs unless it was caused by negligence in which case some companies have a clause that the excess for insurance is paid for by the employee.
Normal wear and tear including dents and scratches caused by stone chips are in general accepted by the lease company as unavoidable.