I sewed each side at the shoulders, then sewed the inside edges together.
Then I stitched the armholes, and turned it right side out.
Next I sewed along the side seams.
This vest is going together quickly, which is great since I want my son to wear it this weekend, and he loves when I sew him stuff. So far it looks like it will be a bit big but not so big that he is drowning in it.