A whole floor plan might spark more ideas.

From what I can see, I might be tempted to capture more of the hallway and include the vestibule/hallway in the bedroom. Otherwise the vestibule/hallway is just wasted space.

And I wouldn't want a closet that's only accessible from the bathroom. I'd try to have the closet accessible from the bedroom.

IMO the vestibule makes no sense. I agree access to a closet from the bathroom is always a bad idea. I would need to see the closet too. That appears to be a very small master bedroom too.

I am actually really excited about the vestibule because it makes the bedroom more isolated which I like. The question was more regarding how a bathroom with closet access would fit in the open space. Any thoughts there?

Can you post a pic of the entire space you have to work with, with the original floor plan. and your proposed changes? From what you posted I can not tell what is the existing master or extra bedroom you have to work with.

I posted an updated one with more info! https://www.houzz.com/discussions/5882212/how-should-i-layout-my-master-bathroom