I find j. crew sizing on dresses, skirts, and pants to be difficult. The dresses seem to run small and shaped for very slim rectangle/hourglass shapes. WIth the fitted sheaths, if the waist area fits me, the bottom is always too big, and often the top as well. Just sayin'. I think the dress you thrifted is a different shape and has a more forgiving, less rigid fid than the more tailored items they carry.
You can definitely try them out to see, but don't do anything on final sale until you feel comfortable that it will work for you!