I’ve used this combination of yarns twice before for this delightful design and both vests came out beautifully. The stitches do relax with this yarn combo and the garment increases one size.

One garment works up in a couple evenings.
To go with Wisteria Textured Hat.

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).

The designer is very attentive upon query.

What I learned making my first Holiday Slipover:
(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 result in 8 or 9 for total length as written. If one continues 8 more rounds, it gets too long. The increases for the front and back pieces needed to start 8 rows sooner.

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.

Another tip:
Arm opening ribbing suggestion:
The lower edge of the arm opening ribbing can stick out somewhat and not all yarns, especially acrylic, 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.

Elected to work 3 rows ribbing for neckline and arm opening edges.
Worked a regular stretchy bindoff.