When I first read your question I was thinking ramp at home, did not see "van."
For a handicapped accessible vehicle, the interest will (still) never be deductible. The cost of the vehicle is a medical expense in the year you bought it. (Because you either paid cash, or you took a loan that you promised to repay, it is considered that you paid in full when you bought it.)
Specifically, the cost you can deduct is not the whole price of the van, but the extra cost of the accessible van compared to a passenger car you might have bought for yourself if you were not mobility impaired. This would be an itemized medical expense deduction on schedule A, subject to the 7.5% threshold rule, so you might not get an actual benefit depending on the cost and your overall income and deductions. If this was 2015 or later, and you did not claim the deduction when you originally filed, you can file an amended return for that year if it makes a difference.
But you have nothing to deduct this year.
