In my pattern Lizzy, you work the body of the sweater like a big rectangle. Then stitches are picked up along each side and seamed together with the 3 Needle bind-off technique and that is how it starts to look like a vest! Below is a photo tutorial of each of the steps involved in the Finishing section:
Lay piece flat with RS facing you and then fold it in half horizontally with fold on top.
Measure 6.75(7, 7.5, 8)”/17(18, 19, 20) cm down from fold on each side and place a removable marker on either side of the garment at that mark (note: you are creating your armhole here so if you would like a shorter or longer armhole depth, simply pick the measurement you would like and place your marker there).
Lay piece flat again with RS facing. You should have 4 stitch markers placed.
Starting at bottom right edge and using your US 8/5 mm 32”/80 cm circ, pick up and k 3 sts for every 4 rows up to the marker (in my sample, that worked out to 46 sts – if you didn’t adjust any measurements your picked up number should be around the same). Remove marker, cut yarn and let the sts rest on the circular needle.
Now you will need to pick up the same number of sts on the top right. Using your extra US 8/5 mm 24”/60 cm or 32”/80 cm circ and starting at the marker you placed at the top right of the piece, pick up and k 3 sts for every 4 rows to the end, making sure to pick up the same number of sts as you did at the bottom right. Remove marker and do not cut yarn.
With RS’s together (so WS facing you), fold piece in half again so that your needles with picked up stitches are one on top of the other (extra needle with working yarn on top).
Using one of the empty needles from either of the circulars, work the 3 Needle Bind-Off Technique (see “Techniques” section for detailed instructions).
Flip work around so the RS is facing you again and the unworked side is now on your right
Rep steps 4-7, making sure you pick up the same number of sts. Then weave in remaining ends.