Sew the buckle on the belt, I recommend by hand so you don't break the needle near the metal buckle. Just thread the buckle on the right belt end, fold over, and sew it on. You may want to move it in more, if you know you'll need more belting action. Alternatively, pin it on, try on the vest, and then determine how far to sew it on. Also, those seams you basted, partway in from the edges of the liner? Sew those on more thoroughly, through all the layers of fabric. Go over a few times, since there is a lot of stress there.
Mark the places where buttons and holes go from the liner. I went with holes first. If your sewing machine does holes, great! If not, do a quick google search on how to do it by hand, and use the crazy old contraption that replaces the shoe on old sewing machines that moves the fabric around to make a hole.