I asked my chef wife and she had never heard of using vinegar in a fondant.
She did say that most of the store bought fondant has glycerin in it to make it more pliable over a longer period of storage.
We make fondant with powdered sugar and
water, mix well and cook to melt the sugar then kneed to bring it into softness, best if used within a day, to store it overnight wrap it in plastic wrap and put in the fridge.