vid is assigned on a chip by chip level at the factory.
it is the value that is detected by the mainboard and fed to the cpu as it is the unique voltage that is the specification for that certain chip.
vid will of course range, and those in the know can expect higher OC's from chips with lower vid compared to other chips. not true in every case, but in controlled settings lower vid seems to correlate with higher OC's.
with that said the intel site gives a broad range of vid voltages, i was trying to get a real grasp of what that vid range is for clarkdale.