Start with a Provisional Cast on to work the collar in one direction for a few inches. Then turn the work and work the collar in the other direction. Pick up sts around the collar to start knitting the entire vest top down, with integrated collar.
Use Big Merino Cloud (held single) or original Merino Cloud (held doubled).
I used 4 skeins for the small size (just a bit over 3 skeins), so the medium will use 4 skeins, while the Large will use 5 skeins, and the Xlarge 6 skeins. I used color H38.

It’s a fun pattern that is never boring because of the cables that use seed stitches for extra texture.