nothing prevents you from taking 179 or even special depreciation 168(k) on every vehicle. this assumes you have enough business income to use the deduction in the case of 179. there is no income limitation for 168(k) section 1031 - like-kind exchanges no longer applies to anything but real estate.
say the cost of the vehicle is P
you take depreciation [this includes 179, regular MACRS, and 168(k)] say D on the vehicle
you now have a tax basis of TB which is the same as P - D
you sell it or trade it in for SP
if SP is less than TB you have an ordinary loss
if SP is more than TB but less than P you have depreciation recapture - ordinary income
if SP is more than P you have a capital gain on the difference between SP and P and depreciation recapture of D.
there's no way to avoid depreciation recapture except to donate the vehicle to charity.
depreciation recapture never goes away. a $20K vehicle that is fully depreciated and sold after 10 years for $2K results in depreciation recapture - ordinary income of $2K
your benefit from the 179 the first year after that it's sort of a wash between the depreciation recapture on the old vehicle and 179 on the new.
