Arduino + Ardumoto : External power source
No - dont use the VIN of the Arduino- it lacks power to power up that Ardumoto driver.
Use the VIN on the Ardumoto labeled Max18V

The only thing that you will loose because of 1volt shortage is some torque and slightly faster battery depleting.. but its in an acceptable range any way.

The VIN on the arduino is only used to power the PIC and some other controllable devices especially the Serial Communications.
Never connect any heavy loads or voltages to the Arduino. Thats the purpose of the Ardmoto.

Keep the power source of the Arduino and the Bridge separate ideally- or at least isoloted by another voltage regulator if you want ot use one battery pack.