I seems to be just meandering but one site I found likened it to map contours, ie echo quilting. Lainey
This is the basic running stitch, but each stitch is taken in a different direction. It can be a pattern such as a zig zag or completely random.
I found this answer on a beading website.
Hi Heather, It sounds as if you are using an automatic stitch control on your machine as I am not sure what FMQ is. However vermicelli stitch is just a wiggly stitch which never crosses over itself. I usually liken it to contour lines on a map as they never touch. I do it free hand just drawing it with the machine needle. Hope that explains it if not get back to me and I will try to make it easier. Kath
This one on a quilting forum.