How many of us have finished a vest only to find that it's too small at the bustline?  If it's only a little too small, you can open the side seams, butt the raw edges together and join them with 1" wide strips appliqued over the join on the right and wrong sides.  Cut the strip for the right side an inch or two longer to wrap over the armhole and hem edges.  This should give you at least 1" and up to 2 1/2" additional width, depending on the seam allowance.