Can i depreciate a business vehicle in expense category Assets and garner the special depreciation or must I use Vehicle expense cat and claim mileage or actual expenses.

You may depreciate a business vehicle or you may claim the standard vehicle allowance.  See this TurboTax Help.

If you depreciate the business vehicle, the asset purchase must meet the qualifications for claiming special or bonus depreciation.

The IRS imposes depreciation limits for automobiles.  See IRS Publication 463, page 29.

Depreciation Limits 

There are limits on the amount you can deduct for depreciation of your car, truck, or van. The section 179 deduction and special depreciation allowance are treated as depreciation for purposes of the limits. The maximum amount you can deduct each year depends on the date you acquired the passenger automobile and the year you place the passenger automobile in service.