Generally you don't. If your van has been fixed properly, and is in as good or better condition than it was in before the accident, how can there be any loss of value?
But...insurance does recognize three types of diminished value.
Immediate Diminished Value is the difference in resale value of a vehicle immediately before damage has occurred and immediately after damage has occurred (prior to repair). Most jurisdictions (courts) will use this standard as the primary measure of damage when courts are employed to seek reimbursement for damage from a negligent party. As courts are rarely the chosen venue for recovery of property damage, the standard of “Immediate Diminished Value” is rarely employed in resolving Diminished Value Claims . . .
Inherent Diminished Value assumes optimal repair quality has been achieved and is defined as the amount by which the resale value of a repaired vehicle has been reduced simply because the subject vehicle now has a significant damage history. “Inherent Diminished Value” is the most widely recognized and accepted form of Diminished Value. It is also the basis upon which any supplemental form of Diminished Value would be added. A common “Supplemental” form of Diminished Value is “Repair Related Diminished Value” . . .
Repair Related Diminished Value includes any additional amounts by which the resale value of a subject vehicle may be further reduced because of less-than-optimal repairs. This could include anything from minor cosmetic imperfections to major structural defects.
How Do I Collect Diminished Value ?
It is universally acknowledged, in every state, that Diminished Value is owed by the liable (at-fault) party that caused the Diminished Value damage. If the liability insurance carrier for the at-fault party owes for repairing your damaged vehicle, they owe for the Diminished Value as well - It’s just that simple !
While it has been universally accepted that Diminished Value is owed to not-at-fault victims (3rd party claimants), courts are generally taking the position that insurance companies do Not owe Diminished Value damages to their own insureds under their policy’s Collision or Comprehensive coverages. However, there are three states where insurance companies Do owe Diminished Value to their own policyholders – Georgia, Kansas and North Carolina . .
So unless you happen to live in one of those states, there isn't much chance you will get diminished value.
You can of course immediately sell your van after having it fixed, and if the only offers you get are below fair market value, claim that difference is diminished value and attempt to collect from the person, but if you do his insurance will represent him in court and there isn't much chance you would prevail.