Dependent on the strobe type and power setting, the sync speed may not be the "actual" sync speed. Though the body max sync's at 1/200, if you have wireless vs wired sync, full power vs 1/2 power, Broncolor vs Buff, etc., your sync speed may be well below that.
I've done my share of dance shots, for a few theatre companies around here. I shoot Broncolor, with Pocketwizards on a 1DMkIII/1DsMkIII setup. Yet, I still set my shutter speed to 1/125 or 1/60 and let my lighting freeze the subject's motion for me. If the ambient is low enough in the room (model lights to low ratio), they won't influence your shots at all.
The vignetting I see looks more like uneven illumination of the scene, but since you've revolved around the camera body, it looks more like the curtain getting in the way of the strobe peak/fall.