Join Date: May 2020
Location: mays landing nj.
If the vaulted ceiling also gets crown molding with the rest of the other room being flat, it is possible to make the flat and the slope converge to appear to be the same. I have done this twice I think in the past. I had to get the information from my old boss who had retired 10 years before. It involves a small triangular piece of crown and also the sloped crown must be beveled back at the ceiling with a block plane.
I do not remember the details, maybe a website or You Tube site may help. All I do know is it can be done ,