You now have a nearly finished vest and are finally ready to sew those shoulder seams. Whether you have lined the back or hemmed it, you will finish these seams the same way. If you lined the back, treat both layers as if they were one.

With right sides together, pin the back shoulder edge to the front's main fabric. You will probably want to sew with the front of the vest on top so you don't accidentally catch the lining under the needle. This means pinning the pieces together with the pins tucked under the lining. Sew from neck edge seam to armhole seam. Trim this seam allowance, and tuck it and the lining seam allowance inside the front between the main fabric and the lining. Press. Blind-stitch the lining's shoulder seam to the back the same way you did the side seam.