Narrower seams will affect how the garment fits. Velcro is about 3/4 of an inch. I would take a piece of fabric, about 2.5" wide, the length of the back seam, plus and inch or so. Fold it in half the long way, wrong side out. Sew across the narrow end, turn right side out. This will give you a finished short end. Sew the strip to one side of the back seam, with the finished side about 1/2" below the raw edge of the neckline. Take a half inch seam. Now you have a sort of a placket to attach the Velcro and the item will still fit. Personally, I would add snaps, not Velcro as Velcro is very stiff.