I have the same situation in my house only it's my living room that has the vaulted ceiling. We put crown moulding in every room except the living room. I had worried that it would look unfinished but it's fine and I've seen several million dollar lottery show homes do the same thing.

Are you planning on doing this yourself? Working 20' up in the air isn't something I'd consider. We hired painters to do the highest walls .... ain't no way you'd get me that high up on ladder. :O

No, I was not going to do it myself. We need a master carpenter to handle this job. Cutting molding on a sloped ceiling is not easy. Yes, it does look unfinished. Because the room flows into the rest of the house where we have crown molding. Cut and paste my link and look at the pictures I referenced. You will see what I mean. Our house is very similar in design to these pictures.