After the neck & collar we knit the arm holes.
Pick up stitches on 4-5 knitting needles. I have 4 x 16 + 12. All together 76 stitches on both sides. Then knit 2, purl 2. Repeat at 5-7 rounds and cast off.
After knitting the arm holes you need to turn the vest inside out and weave the loose yarn ends there.
The arm hole ribs should be a bit tighter than the holes because then it prevents it to get loose and fit better under your winter jacket slim fit arms.