Work each side of neck separately
Decrease one stitch on neck edge every other row twice (ie four rows knit)
To do this on left shoulder on right side knit 1 SSK knit to end every other row twice. When working on right shoulder knit to three stitches before neck, K2tog knit 1 on right side every other row.
Knit one row, break yarn and keep remaining 15/17 stitches on stitch holder.
Rejoin thread and work front as back up to the armholes. Armhole shaping is same as front.
Round neck is worked over 20 rows for small size, 25 rows for medium. This means for the small size start neck shaping on 23rd row from the end of armhole shaping and for medium size start shaping 18 rows from armhole shaping.
Knit 24/26 stitches, cast off 21/25 using stretchy cast off, knit 24/26 stitches.
Working on one side at a time cast off two stitches at neck edge twice then 1 stitch every other row until 15/17 shoulder stitches remain. Work to match back in row count i.e. about 4 straight rows before joining shoulder for small size and 11 rows for medium. Make sure front and back match exactly in row count.
When front shoulders are completed either keep stitches on your long circular needle (which is what I do) or transfer to smaller DPNs. Turn the vest inside out and join the shoulder together on the reverse using three needle cast off. Although it is the reverse, i.e. purl side, I use a knit stitch to cast off. Tip: If you work towards the neckline the loose threads will be easier to sew in.
This is how I pick up edges for the neatest look. I use a much smaller and sharper circular needle to pick up and I pick up every single edge stitch starting in middle of the bottom of the armhole. When I have completed picking up the round I place a beginning of round marker on my short cord 3.75 mm circular needle and knit the stitches from the smaller needle and knitting every 4th and 5th picked up stitch together. Work 4 more rows in stocking stitch and then cast off with a stretch cast off.
NB I do not give an exact stitch count but if you follow my instructions you will be fine!
To do stretch cast off: knit first stitch then place knit stitch back on left needle, knit two stitches together through back of loops, place knit stitch back on left needle, knit two stitches together through back loops and repeat until you have cast off the whole edge.