Jack of too many trades..
Join Date: Aug 2013
Location: St. Louis
Personally, I'd go another direction entirely. Put a fake beam, or faux rafter edge against the vaulted part and you'd have something to end your crown against.
Another choice that might give you better options would be to use a smaller width of crown, then you'd have much less "problem" to solve.
I read an article several years ago about crowing a greatroom. The solution was a pretty complicated-looking cut where the crown "wrapped" around the angle. hard to describe, and I doubt it would work here anyway