AfterEffects caculates the ratio incorrectly. Many threads about the how come and the superior method Vegas use and how to get around the differences. Not sure exactly what you would search for however.

Anyway what should work is when you bring the project back into Vegas, go the end of the file click on the FX pan/crop button. When it opens right click on your video in its work window then apply Match Output Aspect at the first keyframe. Chances are it will fall into the overscan area anyway even if you do nothing. Doing this should get rid of the borders.