I thought it was silly that Otto put an invisible zipper in this. As PP said, velour is stretchy and it creeps, making it hard to sew in a zipper evenly. I also didn't want to put a lumpy zipper in the back of a dress for a tiny baby (who will be laying on her back a lot) when the dress is otherwise so cozy and comfy.

I ignored the zipper. I omitted the facing, cut the back out on fold, made the neck a little bit bigger, and just put in a ribbed tee-shirt neckline. There is still lots of stretch in the dress so it can still be put on easily, though it looks a bit less fancy.