NO vaulted ceilings ANYWHERE in a bathroom.

I think in certain spaces, a high part in the ceiling only emphasizes that there are low parts, nothing more. If it's flat you don't pay attention to it. If it does a bunch of different stuff, you do pay attention to it.

Are you sure the stucture you see inside is not just to bring that part of the ceiling down for the lighting IMO rip off all the drywall ans post a pic of that space when it is all visible. I have no clue as to the reason for keeping a vaulted ceiling in a bathroom

it depends on the over all style of the bathroom and the size.

if the shower has a rain head feature, I would do a flat ceiling in the shower, otherwise vault it.

a big open bathroom is great. use it to your advantage with statement lighting, etc.

Do you need to cover that window for privacy?

what are your plans for the other finishes? Two sinks?

Castellated up and down ceilings always look awful.