OK, I doubt this will be any help, but I think I've had similar experiences. . .
I'm not sure I can tell from the pictures what you're talking about. . . but I think I can tell from your description exactly what you're talking about.
That is precisely what I've noticed happens a lot of times with my vinegaroon: the black fades to gray when the leather is flexed. It turns a nice dark black when a finish is applied, but quickly turns gray when the leather is bent. In an attempt to test the durability of the finish, I've scratched it when it's dry, and it turns gray then as well (unlike a dyed and finished piece). I would really like to know why my vinegarooned leather seems to accept finishes differently than my un-vinegarooned leather. . .
Yes, that sounds like what has happened to me. In fact, I seem to recall that a second coat of the vinegaroon does not even necessarily fix this (in my case); oiling should help here, but, as TwinOaks mentioned, be careful not to over-oil. Perhaps the vinegaroon is not strong enough -- though I doubt this -- or perhaps it simply does not work as nicely on some leathers as it does on others. Or perhaps it's something else. . .?
In the end, though, I would have to say I still like the vinegaroon better than dye.
L'Bum