Once you have experience designing vests made from non woven fabrics which don't require finished seams, you can expand your doll clothing pattern making to making patterns for vests with sewn side seams or patterns made from woven fabrics which need finished edges. To adapt your custom sloper patterns to vests with seams, work out the shape of the finished pattern piece from your sloper, then cut a larger pattern piece with a seam allowance of at least 1/8 inch for smaller dolls, and up to 1/4 inch for larger dolls. Remember when designing the pattern to also allow for the thickness of the fabric, the looseness of the fit, or the clothing the doll will be wearing under the vest. (See photo above for the seams added to the back and shoulders of a simple vest piece). Consider where you can 'turn' a garment inside out (along a side or waist seam) and sew two pattern pieces of fabric right sides together, leaving the opening edge open and the shoulder seams sewn shut. Side seams and bottom seams are often good areas to leave open when turning double sided garments. When you turn the pieces right side out through the hole you leave in the side or waist seam, you can press the vest creating finished armhole, neck and front edges and use fine handstitches to sew the opening you used to turn the piece closed.
The simple one piece pattern used here to make the felt or ultrasuede vest works well for two sided vests made from lightweight woven cotton lawn, even for dolls as small as 1:12 scale. To finish the underarm and neck seams on woven fabrics you will either need to turn over and stitch down a hemmed edge, or finish the edges with attached trims which stop the woven fabrics from fraying and cover the edges to protect them. Use the seam lines you build into patterns to create interesting patterns or changes in fabric that add interest to your doll's clothes.
Study full size clothing to see the variations of vests used for traditional and modern clothing. Many adaptations work well for doll's clothing. Tabard vests can be tied at the side seams with fine ribbon, tiny bits of lightweight velcro can be used as closures for front opening vests. Appropriate width ribbons can be used for tabard vests with 'sleeve' extensions.
Remember that every project you try is a project where you learn new skills. Have fun!