1. This pattern already has the seam allowance drawn in, but if yours doesn't then draw them onto the pattern.
2. I decided to reuse the front placket area and the bottom hem, but you can reuse whichever two sides you want. I was able to reuse the the pockets along with the buttons and buttonholes. Let the seam allowances hang over the two sides.
Then let your boy revel in his new look just like daddy! He'll look so grown up and so dapper with his new vest. It's even cute with a t-shirt and shorts. Henry is wearing an upcycled v-neck tee.