No, the downpayment itself is not considered a deduction.
However, the better news is you can use the entire amount of the purchase price to calculate a depreciation deduction for this business asset. If you are allowed a Section 179 Deduction, then the entire purchase price will be fully deductible in the year of purchase. This is even true if you have a loan for this vehicle.
For more information on Section 179 Deduction:
https://ttlc.intuit.com/replies/3302228
