A very clever design that is well written.
Simple to execute, however, certainly not simple to have designed.
from the designer:
The model in the photos is wearing a size XS, bust circumf 32 1/4” (82 cm), height 5’ 4” (164 cm).

Hygge yarn has a mohair-like halo that’s really pretty.
Dreamy yarn gives the fabric a lovely sweatery texture.
This yarn combo relaxes after washing, so the garment increased one size. Not a problem.

Mannequin picture:
The shirt under the vest, with the traditional batik print, is from Indonesia. The recipient of the vest (real girl in the pictures) is a visiting student from Indonesia.

Gauge: 2 str tog, 9 st/4” after washing.

Made size S with finished dimensions of size M.
Made front / back / lower rib trim in one day.

The below discussion is based on personal preference and is not a criticism of the design by any means. One can make the top as written with great success.

Side area issue:
The design vest has about 16 rows after the join of front and back. The directions as written work 8 or 9 rows before the ribbing. If I had continued for 8 more rounds, it would have been too long. The increases for the front and back pieces needed to start 8 rows sooner. My row gauge was accurate.
Others have noted this issue. But since this was the first time on this design, such a top down adjustment was too tricky for me.

Modification for raising arm openings and adding to under arm / side area depth:
Back:
start the increases after working 8 fewer rows designated for your size after the beginning purl row.
Example: size S says to work 28 rows st st.
Instead, work 20 rows st st, then commence with the increases.
Front:
start the increases after working 8 fewer rows designated for your size upon making the R and L shoulders.
Example: size S says to work 15 r st st.
Instead, work 7 rows before doing the increase rows.

After joining front and back, work st st to desired length, which should be ~ 16 rows.
Pick up 16 fewer stitches when working arm opening ribbing.

Arm opening ribbing suggestion:
The lower edge of the arm opening ribbing can stick out somewhat and not all yarns will cooperate in lying properly.
Pick up stitches as written.
Ribbing:
r 1: dec 1 st at beginning and ending of this row via k2tog or p2tog as needed. (2 st dec)
r 2: work 1x1 ribbing around to last two stitches, dec 1. (1st dec)
r 3: dec1, 1x1 ribbing around. (1 st dec)
work additional rows as desired, then bind off as written.

Modification:
Neck band ribbing: 5 rows only.
Arm opening ribbing: 3 rows only.
Worked JSSBO Jeny’s Surprisingly Stretchy Bind-off in rib pattern with US 11 for neck band and US 13 for arm openings.

made in just two days!
My first finished project for 2022!
Started December 30, 2021.
Finished January 1, 2022!
Happy New Year!