My experiences of repairing torn technical fabrics have been best with riptop tape. Not pretty but it works. The issue is spreading out the load, which must be fairly high along that seam, and as we can see, there's not much adhesion between extremely slick tech threads. I'd fold it over the bottom of the garment and run it about 1" up both sides of the gilet's main body. I'm a lousy seamster, so if you're like me, you could try the dry cleaners and then the ripstop tape if their repair fails.
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