I think I figured it out. The only way I could make it go away was to turn off feathering altogether, but that wouldn’t do because the backgrounds didn’t mesh together that. But I realized that the problem seemed to be that I was using the “increase gamma” effect to lighten the top layer, and, for whatever reason, Sony Vegas doesn’t seem to recognize effects for the feathered portion of the video, so the “shadow” was just a part of the original, darker video showing through. I solved the problem by rendering the video as a new mp4 file with with the effects, but not the mask, then I applied the mask, with feathering, to the new mp4 that had no effects.