It’s subtle, but there is a difference 
Here’s some more screen grabs.
From the PGs, you can see at the horizon, where the water meets the skybox, without a vignette the water blends well into the skybox as it’s reflecting the white horizon above. But with the vignette, the vignette color of black seems to be tinting the reflected white of the water MORE than the white of the skybox horizon, so that there’s a stark difference between the water and sky (it’s no longer well blended).
And here’s what it looks like in my application. You can see a clear line at the horizon where the water plane ends after a vignette is applied.
If the vignette is a true POST process it should be treating all scene pixels the same, not tinting some materials more than others.
I suspect there’s something about the water material that is getting rendered or blended AFTER post processes are applied, but I don’t know for sure.
I hope this makes things clearer.