I also would like an answer to this question. It seems to me that "nebulous" gets its meaning from "nebula" and therefore would imply the unclarity of indefinite, or not well well defined. Nebulae are called so because they are cloudy like, but not so much in the sense of hard to see through as much as not well defined edges and such. "Nubilous" on the other hand is cloudy or misty as in cannot be clearly seen through, or hard to identify objects in. They both seem to have similar etymologies, but one took a cultural detour through space. That's my guess.