The dress is constructed in two layers: a knit underlining and a stretch lace overlayer. They're sewn right sides together, then turned, then topstitched in two parallel lines. The back neckline is round, the front neckline higher and also round.
I sewed by hand, making first a line of stitches close to the edge to close the seam and catch the stabilizer, and then another row of stitches running parallel. I used a backstitch for strength with the knit. When I got to the center of the V, where the seam allowances necessarily get quite shallow, I shortened my stitches and whipped around the V a bit. If you're using a machine, just shorten your stitch length at the turn of the corner, starting approximately where I started using red pins instead of white.
See how much better the V neck looks!
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