When you use neckties in clothing with defined shapes such as a vest or jacket, the ties will need some support.  The easiest ways to get that support are to use a foundation (like muslin) or fused stabilizer.  You can piece to a foundation with the sew-and-flip method, then cut a pattern piece from that.  Another option is to stabilize the stretchy tie pieces with a very light-weight fusible interfacing and use them as you would any stabile fabrics.  For garments that need more structure, you can layer and quilt the pieces for more support before constructing the garment.