I have done this technique to create a simple vignette to soften edges in aftereffects and since upgrading to Aftereffects CC I am having some problems with the edge of the mask giving me a horrible hard edge. It does not make much sense to me. Also the same problem when adding a radial ramp to a soild layer, am i missing somthing simple?
to create a simple vignette i am....
Creating black solid - add mask - subtract mask - set opacity - add mask feather.
Am i missing any simple setting that have changes since updating from CS6 to cc?
Changing the opacity can cause banding issues, so without seeing settings or images, that is probably what I would nix. Instead of choosing such a dark solid maybe try a lighter gray. Another technique would be to use a blur on an unfeathered edge and increase the blurriness to increase the feather. Box blur is probably best, as you can repeat the edge pixels. I don't know if that solves your problems, but let me know if I can provide any other ideas or advise.