Then, I unpicked the stitches that held the vinyl to the zipper. I also removed the zipper stops and repositioned them at the new "top" of the zipper. All you need for that is some needle-nosed pliers. See the underflap? That is a thicker piece of vinyl that I shaped easily with my scissors. Then I just folded under the vinyl and used a zigzag stitch to "hem" the boots.
I used a 110 leather needle with my leather rolling foot. I also sandwiched tear-away stabilizer between the vinyl and the machine bed so it wouldn't grab.
Even with all that, it is hard to get even stitches. All of the stitches shown below are of the same stitch length. When they are closer together the vinyl was getting bound up. When I grabbed it and started pulling, the stitches got farther apart. This is my test sample.