Here's the way i'd do it. Right click your curve, select vertex, right click your square select vertex. Select the cv you want to snap, hold down C and middle mouse click and drag on the "squares curve" until you get the position you want. I always find when you actually try to drag your actual CV it's difficuilt to get it snapping. It's easier to select your CV, hold down C and middle drag on the curve you want to snap to.