This listing of one by Anthony Canney shows that he has a sort of "keyhole" gap above the lacing... perhaps this is a construction point rather than an aesthetic one?

www.etsy.com/listing/86996172/the-dueler...unk-corset-vest-with

I agree that you should re-lace the back so the edges are parallel, then pin out the excess fabric so it will sit that way in the final version. You may find it very helpful to use several shorter laces, rather than one long one (a lace from top to shoulder blade point, a lace from there to waist, a lace from waist to bottom edge, for example) to keep tension in one area from disfiguring another.

You may want to consider a back shoulder dart, like you often find on women's bodices in 1950s patterns. The dart starts at the top of the shoulder (on the shoulder seam) near the neck and the point is 3-5" below. You only need to take out a little bit - maybe an inch? but it can help provide freedom of movement in the arms and shoulders while keeping the shoulder seam trim and smooth against the body.

While you're working on the shoulder area, it would probably be prudent to just take the shoulder seam out completely. Lace it on over a t-shirt, so you can pin the loose shoulder pieces up where you want them, and work on the front and back separately. Then when you've got them nice, don't just force them to meet but smoothly grade the edges from front to back. That will keep the fronts from pulling on the back, and vice versa.

i have made a number of mens corset vests. What i have found for my clients it that getting the lacing to stay evenly spaced has to do alot with the pattern itself. It looks to me like you need to haave less fabric in the upper back and shoulder. I have also found that having steel boning (i use flat) on both sides of the eyelets makes a huge difference in maintaing those lines

I agree with Marion. But is this is a mock-up or a real thing? I can see you you have already put the eyelets and can't change the back edge on this one unless you want to replace the back panels. I also think you have too much material at some points of middle back panels and it's causing misplacement of the shoulder seam (you can see the end of the shoulder is pulled up).

I would suggest to try to lace it so the gap at the back is flat and then try to determine the excess of the fabric on the panels by pining it. In that case you might even notice they are too long so you will need to take some on the shoulder seams.

It is actually the plan to use fan lacing on the final corset, once I get the rest working

here is another example: www.korzety.com/?page_id=307/