Designing Kitchens and Bathrooms - Cement board/remodel questions

I just took down the tile in my bathroom. It was adhered directly to the plaster, which I also took down in the shower enclosure. I'm planning on taking the plaster down in the whole bathroom. On the outside wall, the damage to the underlying plaster was pretty significant, with some damage to several studs. Replacing the studs will be no big problem. Once I replace the studs and the insulation, I will need to put up the cement board. I've read several threads and I'm unsure if I should put up a vapor barrier before putting up the Hardiboard. If so, should I put it up around the whole room? And does that include the ceiling over the shower/tub enclosure?

The thickness of the plaster substrate I removed was about 3/4". In order to closely match this, would it be a problem to use the 1/4" Hardiboard in the entire bathroom, then cover that with 1/2" hardiboard in the shower/tub enclosure, and with 1/2" sheetrock in the rest of the room? I would be tiling in the shower/tub area and painting the rest of the room. I want to make sure there is enough thickness to get me to the flange on the tub.

Does this make sense to do? The plaster in much of the rest of the bathroom seems to be in decent shape except for the thinset from the tiles I took down. I can scrape it off without a whole lot of difficulty, but I don't want to harbor any potential mold or mildew underneath it. But I fear that matching the thickness at the seams would be very difficult, which is why I was thinking of the dual materials. Would it make more sense to use 3/4" sheetrock in the rest of the room?

Am I overthinking all of this?

Thanks in advance,
Trevor

Trevor: No, your thinking is great. Why not think about putting a vapor barrier and only one layer of 1/2" backer board in the shower area, but on the rest of the non-wet area use 1/4" plywood, then 1/2" sheetrock to fill out to the other walls. It will be somewhat cheaper and easier to apply the sheetrock to the plywood rather than to the cement board. And in the shower area, if you are replacing all three walls, it won't matter whether or not you make it the full 3/4", since there is no reference point inside the shower. Post back if we can help further.

Another question to add to this post - I've seen that sheetrock is fine for the ceiling above a shower. Is this still true if I'm going to tile the ceiling? Or should I use the Hardiboard on the ceiling as well? I believe my ceiling joists are 16" OC, so I would add extra blocking for support.

And about the vapor barrier, what is the recommendation for a vapor barrier under hardiboard and tile? I have read different things from different people. One wall in the shower is an outside wall. I live in Massachusetts (Cold winters).

Also, a clarification - my tub/shower is at one end of the bathroom (9'4" x 5'), so there is a transition on each long wall where the Hardiboard would meet the drywall. Is it safe to assume that in this case I'd want to match the thickness for a smooth transition?

Thanks,
Trevor