Does the VCC Output pin always give out a consistent 5V Voltage supply? What is the current output?
If you power from USB the vcc pin gives you the voltage that USB is providing. Somewhere between 4.75 and 5.25v.

If you use external power it provides 5v since that is what the regulator provides.

The current limit is either the limit of the usb port or the limit of the regulator on your board. Check the datasheet for that regulator (hint: read the schematic to find the model of the regulator).