Is the Uno Vin pin protected by the voltage regulator? I guess it's all in the title. When using Vin as input, does it go through voltage regulator, or must I feed it precisely 5V?
If you look at the schematic you'll see Vin is connected to the input of the voltage regulator. Just like the barrel-jack (except for the diode).

So the same input-voltage of    7-12V applies to Vin.