Okay! You've all been very patient. Here's some real progress! I've still got a few things to do, but most of it is there.
First, here's my vest. (I'll post pictures of me in all this stuff when I can drag somebody over to take them for me. That'll help with the tweaking.) I used an actual vest I have as a template, then made a mockup and adjusted it a couple of times before calling it good enough to use for a pattern. Princess seams are a pain, but it turned out pretty good. The zipper will be covered by the breastplate, but I could add a velcro'd placket to hide it if necessary.

For the armor, I decided to start with the easier shapes - arms, legs, shoulders. I got rough dimensions by making a diagram (see below for my messy plans) and measuring myself (width at top, width at bottom, length), then transferred the resulting measurements to the posterboard in pencil. I cut them out, curved them, tried them on, made adjustments.

Here's a bit of process for the breastplate and back plate. I didn't want to waste my posterboard with experimentation, so I taped together computer paper, taped it in place, and sort of shaped it, taped that, marked it up, cut the darts out, taped them up again, tried it on again, marked it some more, etc etc. For the back, I taped paper onto an old duct tape dummy that I made a few years ago (but couldn't use for most of this because the duct tape has pulled apart over the years and is no longer accurate - I guess you could say it's let itself go!) and drew the back plate on it. Then I cut that out and pinned it to the back of my vest, put it on, checked the reflection in my two-mirror bathroom, took it off, added more paper to where it needed adjustment, tried it on again, and decided it's pretty good. I'm not totally sure about the length on these, because it depends on how low I end up wearing the belt; I'll cut off the excess later when I figure out the belt.

Here is all the stuff I've done, transferred onto posterboard! Some of it has the detail bits marked up (generally speaking, I'm planning on 6mm sintra for the base plates, and 3mm where it has a second layer for design purposes), and some of it I didn't bother to draw on. (It's easier to draw straight lines before you shape it and tape it together, lol.)

Here's a closer-up of the pauldrons. I was really excited at how they came out! I'll probably put a bit of foam or something in the space between the plates, for support. In this case, I'm not sure which I want to be 6mm and which 3mm (upper or lower), especially for the really long plate. Is 3mm going to be sturdy enough, or will it get damaged more easily?

I still need to tweak the collar (and add the back half); after talking to Jor Kajil about his Neo plans, I decided a collar plate is a good way to go - it'll still look good when I take off the helmet, and it'll give the cowl a sturdier base to attach to when I put it on. Also,
Canderous has one, so it seems logical that there's one under the cowl for this era anyway.
I still need to do the back of the shins too, as well as the soft cod and boot spats by way of soft parts. And I need to do the helmet, of course. But I'm pretty proud of how it's coming together!