When you pick up and knit the stitches for Vianne’s fronts, you want to pick up one stitch per stitch along the cast-on edge. This allows the lace panels on the front to line up as close as possible with the lace panels back. Here’s where you should be picking up stitches and the direction you should be working in.
Fresh off your needles, the bias of the mesh is pretty extreme, but it will become more subtle when you block out your sweater. Just stretch out your upper back a little to see how the back neckline straightens up a bit when the mesh is more open.