I think that Dominos 25-lb sugar bag (do you recall if it was originally a yellow and white package?) is probably the same sugar I use, e.g. cane sugar, not beet. My sugar may be a little more humid because of its storage conditions, but otherwise the same thing. Just try a bit less vinegar and see what happens.
I have to make another batch soon, and I will do a better job of measuring the sugar in a dry measuring cup, not just approximating it in a liquid measuring cup like I did the first time. (Too lazy to wash two measuring cups!)