maximum current that the arduino will draw from the Vin pin?
The limit is when the on-board +5vdc regulator decides to shutdown due to over temperature or over current. Usually the temp is the first limit because there is not much heat sinking given to the regulator in most Arduino designs. The amount of heat being generated is a function of the Vin voltage and the current drawn from the regulators output, the higher the Vin voltage the sooner the regulator will reach it's safe temp limit and start to shutdown. However there is also a minimum Vin voltage (called minimum dropout voltage) required to insure that the regulator stays in regulation, 7.5vdc is the traditional lowest regulator input voltage required for the classis 7805 regulator, but other newer regulators offer a lower drop out voltage specification. So running the Vin voltage as low as possible will allow the maximum current to be delivered by the +5vdc regulator.
All that said, if you are just trying to come up with a current budget for the main power supply current, then planning on 300ma max is probably as good a value as any. The actual on board components (regulator, AVR chip, misc resistors and LEDs, won't draw 100ma on there own), however what you draw from the processors digital output pins depends on what they wire to and how much current the external circuit/components draw. The AVR chip it's self has a output pin max current of 40 ma and all pins +chip overhead has a 200ma max rating.