Disapproved VCs

[Barrett]

Is there a source of info on the number of VC recommendations disapproved in either WW? Reasons for disapproval other than lack of witnesses (BB excepted of course!) I'm curious as to the administrative process--at what level disapproval typically occurs.
By way of comparison, the Medal of Honor in some cases has been approved, disapproved, and the latter over-ruled sometimes decades (or a century) later. Very political.