The problem with a car's electrics is the risk of large voltage spikes from the
starter motor and suchlike - so its wise to ensure a power supply is able to
deal with these. If you can simply use a purpose-made USB charger you save
yourself all the complexity. Simply connecting the battery positive to Vin
is fraught with risk and isn't recommended.
Its normally a good idea to have some RF filtering, a TVS or similar to tame
overvoltage spikes and a good regulator chip or DC-DC converter that will
handle a wide range of input voltages - expect the battery voltage to drop
to 6V or so on starting, upto 14V during charging, and various spikes and noise
can also be present (especially once the battery itself is getting old).
And of course a fuse is mandatory.