We purchased a wheelchair accessible van . Is this deductible

Is the purchase of the Van deductible - No.  However, the costs for modifications to the van to make it wheelchair accessible are a medical expense which would be deductible.  See IRS Publication 502 page 7 - https://www.irs.gov/pub/irs-pdf/p502.pdf#page=7

Car 

You can include in medical expenses the cost of special hand controls and other special equipment installed in a car for the use of a person with a disability. 

Special design. You can include in medical expenses the difference between the cost of a regular car and a car specially designed to hold a wheelchair. 

Yes, it is entered as a medical expense deduction.  The amount of the deduction is the amount you paid, less the cost of a similar van that is not handicapped equipped.