How to use unregulated VIN voltage?
The VIN port is connected to the barrel jack. It must only be used if you are powering the board through that jack.

To access the USB power just use the 5V pin - it is directly connected to the USB.

However, the USB, although the socket it is plugged into can supply more, is limited to 500mA. This is because of a 500mA auto-resetting polyfuse on the USB power input (the large gold and dark green rectangle next to the USB socket - mine has either XE05 or 503X on it depending on which way up you hold it).