This vest is easy to knit based on modular technique started 3 stitch numbers cast-on. I suggest long term gradated yarn for this project like Noro Kureyon.

Size: 2(3,4) years.
Finished Chest: 52(56,60) cm.
Gage: 17 sts and 30 rows per 10cm in garter stitch.
Yarn: Noro kureyon 100% wool;100m per 50g 2(2,3) skeins “shade 319” or 184(200,250)m
Tools: size US 7/4.5mm straight needle, stitch marker, stitch holder, tapestry needle.

pattern errata
3page, Front, 8th section.
Row1(RS): knit to last 3sts, ssk, k1.
Row2(WS): knit to last 2sts, k2tog. 10(11,12)sts.