Ahhhh I am too late :-D I wanted to use your post as a excuse to browse Ravelry for hours, but the pattern from @muscara is perfect. The only addition I have is, that the pattern is worked bottom up and I understand that your problems with your vest is more in the upper body (back and bust) So, if I where you I would knit the pattern upside down, meaning top-down 😂 I also have a wide back and a wide rib cage, but a very small bust line. In combination with thick arms this causes different problems (most patterns which would fit my bust are much to small at my waist or arms). Nevertheless, my problems occur also on the top, so I mostly try to knit top down constructions. For me the classic raglan fits well, but I read that it is not so nice on a bigger bust. But I could imagine you could use another top-down construction where you first knit the back flat, than pick up stitches for the shoulders. When back and front parts are long enough you put everything on one needle and simply knit the vest/cardigan/sweater as long as you like. This gives a lot of control about the back and bust width. As you want a vest you do not need to add sleeves, perhaps just a bit of ribbing around the armholes.
Here is a link with a better explanations than mine:
http://www.basixknitting.com/anatomy-of-a-top-down-sweater/
I guess it would be possible to individualise a vest pattern with this technique.

I hope I don’t wrote something you already know as you are much more accomplished in knitting than me.

And just because I love to browse Ravelry: here are some vests which seem to have a wider back.
https://www.ravelry.com/patterns/library/syncopated
https://www.ravelry.com/patterns/library/alastair-ves