Yeah, I really should have provided pictures, my bad!
But if you look at the underarm section, he actually seems to be wearing two undershirts- the aforementioned compression shirt and a second overlapping one. Originally I thought that was part of the vest, but there's a distinctive ridge where the vest ends around the armhole.
Sorry that all of the lines are messy- I'm using my laptop trackpad ^^;;;
But back to the original point, here we see the armor bit separates the vest from the ribbed section. It's literally sitting above it so it's probably not attached to the ribbed section more than a few tacks. As you brought up, the front middle seems flush with the vest which is the (only) point where I believe it's actually attached, I agree.
And as seen here, that continues all the way around the back. It is very much sitting on top of the ribbing as opposed to being attached to it, but I imagine there was some sort of jointing point to keep it in place. If you'll also notice the bottom where it looks scrunched up, the ribbing has slipped under the bottom edge which also indicates that it's not attached there.
The same thing goes in this one where the bottom of the vest (which is incidentally also the top of the pants- Jumpsuit why!?) is slipping over the ribbing.
There's also the issue of the little strap in the back (the circled bit). It seems to be the connection between the upper vest and the pants that counters the straps in the front. I think it's really only visible in this render...
But the fact that it can loosen up to that extent (as in the photo) while the ribbing section stays taut indicates that the ribbing is moving independently of the connected pieces (bottom of the vest and top of the pants).
I could be completely off the mark and please correct me if I'm missing something (which would actually be really cool since I'm making this way more complicated than it might actually be) but here's my current plan:
Purple indicates the outline of the total undershirt. Pink outlines the sections that will actually have the ribbing, and the orange bit is where it'll be top-stitched to the vest. I'll probably have the shirt sewn to the vest at the main zipper just for ease of putting it on. It'll probably also need shirt garters to keep it down, but that's easy enough. I was also thinking of attaching the extended shoulder bits to this shirt, but I'll need to test that later.
So yeah. That's my thought process. I probably rambled so let me know if something's iffy.
If that's even remotely close to how the actual one worked, I can see why it'd be a hell to wear. Not only is it pulling in very unfortunate places (no wonder his ass looked fantastic :love), it's also looking to be a portable sauna. Woooo :|