On an exterior wall in a heating climate, the poly behind the cement board can act as both the house's vapor barrier AND as the vapor barrier/drainage plane for the shower. To be the vapor barrier for the house, the poly should be continuous and any laps should be sealed. If the insulation in the wall behind the poly is faced (kraft facing, foil facing), render the facing ineffective by slashing it with a utility knife. For interior walls where you just need a drainage plane, the seams in the poly can be simply lapped to shed water, but they don't need to be sealed to prevent vapor drive. With a continuous and fully sealed plastic/visqueen membrane, you don't need Kerdi. If you did use Kerdi, then remove or slash the visqueen to render it useless as a barrier where the Kerdi and Visqueen overlap. You don't want overlapping non-permeable membranes in a wall assembly.

I just recently had to make that same decision...and I went with a Sterling tub and 3 pc surround in my 1930 home. Main reasons: 1. Tile was there before (c.1966) and caused a water leak that caused a quite large piece of plaster to fall from my kitchen ceiling...granted they didn't install tile back with a good backer board. 2. I had to get rid of the 1966 cast iron tub...it was not in good enough condition to keep. 3. I wanted a larger soaking tub...my cast iron was small! The Sterling tub is 18" tall and very wide...and has arm rests. 4. This was the hall bath. In a master bath, I would probably have gone with a tile surround. 5. My budget on this bathroom was small....as the kitchen is coming up in January and that's where I want to put my $ So far, I love my Sterling tub/surround. And since I mainly take showers, my plumber raised the water controls up about a foot or so. I never would have thought to do that....but it's GREAT. Even my 11 year old son mentioned how he likes it better higher. Here is a picture below. KOMPY

I don't believe acrylic shower surround is a proper substrate for tile. Tile needs to be set on a structurally sound platform or you will end up with cracks. Additionally I wouldn't be confident that the thinset could adhere properly to the acrylic. Your biggest cost is going to be the construction and tile work. Do it correctly or you will be disappointed with a failing shower down the road. A new tub is really not that expensive in the scheme of a bathroom remodel: I see several on home depot for under $200. If the contractor is suggesting cutting the acrylic, I would be concerned whether he is doing proper construction and waterproofing. I am not saying he isn't, but that raises red flags to me and would make me concerned.

first off, that is a horrible niche design. Cutting the tile to border the metal edging is not needed. throws off the tile design. if you want a niche, again, do the math and place it where it can fall symmetrically between full tiles. like this one. nice and neat. use a metal edge like a Schluter Jolly for the border, or see if your tile has a liner or bullnose feature. secondly,,,tile layout depends on what the manufacturer recommends. most of those tiles in that size should be placed on a 1/3 offset, not 1/2. it is because of bowing and cupping. some tiles are not perfectly flat. place two tiles back to back (or front to front and see if you see light between them or if they are perfectly flat to each other. Whatever tile you buy, they should have specs that tell you the grout width, and the way to lay them. I do like the first tile w/no offset. but if you want this one above, try for a 1/3 offset layout. there are a few ways to do it. Both of these are 1/3 offset. nice niche above, this one below, meh. should have been larger and fit within that one full tile. And watch those corners. you never want a small sliver cut. The one pictured is ok. I just dislike how they did the niche. 12x24 tiles w/a perfectly placed niche. and the tiles are finished on the outside edges. see how it fits between those tiles w/no tile cuts around it? this one too. placed perfectly between full tiles. you must do the math on a dry layout and tranfer those numbers to the shower wall, placing the niche exactly where your tiles will be laid. another excellent measured niche. full tiles above/below, no sliver tiles in the corner, the niche was bordered on the inside w/full quartz pieces and a mosaic tile on the back. very nice. Outside edging is a type of liner in that same tile design. bullnose edges would also work here