It depends. The expense for a wheelchair van so that you or a family member could safely enter and exit a vehicle does qualify as a medical expense. But, only to the extent the cost exceeds a similar van without accessibility options.
You must itemize your deductions to claim medical expenses. Also, medical expenses are only deductible for the amount in excess of 7.5% of your AGI. For example, if your AGI is $50,000, only your medical expenses over $3,750 can be claimed as an itemized deduction.
Refer to IRS Publication 502 for more information.