I'm just wondering why the VIN would require 7-12 volts when the USB port itself only requires 5 volts? What happens if you power an Uno on VIN with 5 volts?
USB's Vcc pin is assumed to be a stable, 5v supply, and is directly connected to the 5v bus on the Arduino board with no further regulation. Vin is assumed to be an unknown and unregulated voltage in the range of 7-12 volts.
The 7v minimum is a requirement of the design of the on-board voltage regulator which needs the 2v "head room" to maintain its output at 5v. A 5v power supply could be designed with a larger input range, even including voltages less than 5v, but the simple design used on the board covers a range of sources at a lower cost than a higher-performance power supply.
The UNO takes your input power supply (7-12 volts) and then regulates the voltage down to 5 volts. This way, the UNO operates at a constant voltage regardless if your input power supply wanders about from 7 to 12 volts.
The USB port, BY DEFINITION and International Standards, needs to operate at 5 volts.
If you apply 5 volts power supply, the UNO won't be able to regulate to 5 volts. The voltage regulator itself will have a voltage drop, and the UNO will operate at less than 5 volts. The UNO may or may not operate properly. The USB will likely suffer from more problems than the UNO if you apply 5 volts. Good news is that the UNO will not likely suffer permanent damage.